Recently started converting some of my homebrew subclasses from paper onto the site, one of which gets an optional resistance, now I can add a resistance easily enough but I don't know how to make a list of them that enables the player to choose one. 
Another small one the same subclass gets a special cantrip which damage is based on which resistance they chose (sort of like how draconic sorcerer gets to add their charisma to their ancestor damage) is there any way to code the cantrip to recognize that? Furthermore for some reason despite both the "spell range type" and "attack type" fields being filled as ranged and the range is specified when taken into a character sheet there is no button to roll for an attack.

I can't speak to all of the issues, but you can make a homebrew copy of the dragonic ancestry sorcerer to see how they did the resistance selection and such. Copying an existing subclass/feat/item/etc. that does the sort of thing you want is often the simplest way to figure out the homebrew system.

Oooh interesting, thank you I'll give it a look, hopefully that should remove allot of the issues. 

First you have to make the feature that contains the choice of resistance. Then you have to create an “Option” for each of the resistances you included for them to choose from.

Next you have to make some decisions about how mechanically functional you want it to be compared to how much work you want to put into the project.

Most Work:

If you are married to that being specifically a “Cantrip,” and you absolutely insist on that class feature adding the damage to that Cantrip when appropriate and not at any other time then that’s a lot more work. It would basically require you to make:

• Multiple versions of the Cantrip to account for the different damage types. A version for each damage type with the bonus, and another version with all of the appropriate damage types and no bonus damage.
• Another class feature that attaches the version of the Cantrip that can do multiple damage types but adds no bonuses. This feature would also need the same number of Options as the first feature and uses those original Options as the prerequisites for the Options from this feature. (This would result in the Character Sheet having two separate versions of the Cantrip.)

Medium High Work:

You could do with a class feature that does the exact same thing but isn’t technically a spell. Making it as a class feature instead of a Cantrip would otherwise let you achieve the granularity you want without mucking about with all of the Cantrips, but would still require the Options and the prerequisites.

Medium Low Work:

Make the Cantrip once that can do whichever kind of damage but applies no bonus to any of the damages. (Kinda like Hunter’s Mark.) Make a class feature with options and prerequisites that all say the same thing but with the name of the damage types customized.

Least Work: (Just like how Elemental Affinity was programmed)

Make the Cantrip once that can do whichever kind of damage but applies no bonus to any of the damages. Make a class feature that explains the bonus as text and let the player track the for themselves.

(Personally, if it were my project I would go with the “Medium Low” option as it likely has the most efficient  cost:benefit ratio.)

I explained how to properly set up Option trees in the following thread: https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/67361-subclass-help#c4

I hope that helps.

Very helpful thank you, ill have a test around with the medium-high option, if I cant fumble my way through ill try your suggestion. hopefully this works, much appreciated :).

Edit: my only issue with doing it in the sheet was that the damage logically is 1d8+{{modifier.cha}} however that cannot be inserted into the 'fixed value' field meaning i could only do it through creating a cantrip itself. Is there some other '@chamod type signature i can put into that field that will work? 
Edit 2: never mind i got it all working, thanks for the assistance!

True, you cannot customize the spell on the character sheet to automatically add the Cha modifier, but that won’t be changing constantly so the player could put “3” as the damage bonus while their Cha is 16-17, and then when it goes up to 18 they just change the damage bonus manually from 3 to 4. When their Cha eventually hits 20 they change the 4 to a 5. They would have to change it between  one and three times maximum over the course of the entire campaign the same as they do on their paper sheets now.  🤷‍♂️ 

Anyway, I’m glad you got it all sorted. Happy to help.

PS- Don’t publish your subclass yet. I can almost guarantee that you could improve upon how it’s set up (even if it’s just typos) but won’t realize it for a while. Wait until the “Ts” are crossed and the “Is” dotted. You’ll thank me later.

I did actually eventually find how to add the charisma modifier to the damage, I ended up doing it as an action then adding an extra thing to 'modifiers' to add the modifier as damage in there, it's a bit of a dumb way of doing it but it works with no flaws as I can see. 

Also, this stuff is only for personal use for a future homebrew world I am making so I don't intend for it to be published. besides I am getting error messages telling me they are 'too similar to other things even though I'm making a sorcerer subclass about blood magic and it's telling me its too similar to the circle of spores druid, which makes no sense to me at all but hey I have no idea how it identifies that sort of stuff. Anyway thanks again, the majority of searching I did to learn the easy stuff led me to comments from you so you really are a pillar of support that I think a lot of people lean on :).

Yes, adding an action to do the same thing was one of the details I neglected from the “Medium Low” rout I suggested, but I would have personally attached the action to the options so it would automatically reflect the correct damage type instead of just listing it as generic additional damage. (I just wasn’t gonna full in all the blanks until I knew which method you wanted or else that post would have been 4x as long and don’t nobody need that.)

It does reflect the correct damage, I have it set up so you choose which resistance you want then in a different class feature I set up 5 different attacks each with their own prerequisite, so if the player chooses bludgeoning damage as their resistance they get the bludgeoning damage attack. It is working as intended as far as i can tell, but it'll be nearly a year before i run the game with this subclass in and even then my players might not end up using it, so we will see if they run into any issues.