I want to make a homebrew cantrip similar to Green-Flame Blade and Booming Blade but can't quite figure out how to get the damage - and damage progression - to show up correctly on the character sheet. I know these are more complicated than most other cantrips but any advice on how to programme the damage properly would be much appreciated. 

Looks like this was made available since original post?
https://www.dndbeyond.com/changelog/298-spell-updates-and-more
I'd like to homebrew Green-Flame Blade as well and I'd like to take advantage of the new stuff described in that update link.

If you copy the cantrip you will see all the modifiers and settings needed to make it work.

Copy the cantrip? Are you assuming I have a digital version of the cantrip? I do not. That's why I'm asking.

Make the spell.

Spell Level: Cantrip

Spell School: Evocation

Casting time: 1 action

Components: V, M (a weapon).

Spell Range Type: Ranged

Range Distance: As appropriate. For Melee this may be 5 ft. This is for the initial target of the spell. There is no way for this to be based on your weapon or reach.

Duration: Instantaneous

Descripton: fluff, I'm not breaking copyright to provide this. If you have non-digital access then you know what to put if not then this is entirely homebrew and you can put what you like.

Check "At Higher Levels Scaling"

Higher Level Scaling Type: Character Level

Available For Classes: As appropriate for your spell.

Create Spell

In Additional Information:

Check "As Part Of Weapon Attack".

Affected Weapon Range Type: Melee or Ranged as appropriate. This determines the type of weapon that must be used for the spell - note that even if you choose ranged weapons the range of this spell set earlier is used for targeting not the range of the weapon.

Spell Effect Tags: Combat, Damage, plus any other you think is relevant.

Now, we will do modifiers. You will need two modifiers and for sake of simplicity we will consider them "primary modifier" and "secondary modifier". The Primary Modifier is the initial damage done and the Secondary Modifier is the follow-up damage which can be to a different creature like Green-Flame Blade or based on a trigger like Booming Blade. For both spells, the Primary damage does nothing at first.

Primary:

Add Modifier >> Type: Damage >> Sub-type: (your choice).

The modifier will not deal damage or do anything to start with; if you're going by the Green-Flame Blade or Booming Blade routes, that is. At base level the initial damage is just the weapon damage which you don't need to input. We still make the modifier so we can increase it for level progression.

Secondary:

Add Modifier >> Type: Damage >> Sub-type: (your choice) >> (damage details) >> Restriction: (secondary info)

(damage details) would be the damage to be dealt. If going the Green-Flame Blade route you just check "Use Primary Stat" and the damage will be based on the spellcasting ability modifier. Or you can fill in dice roll info or fixed value.

(secondary info) is how the secondary damage is done. For Green-Flame Blade this would be "To Secondary Creature" while Booming Blade would be "If Target Moves".

Now you need the scaling so let's move to At Higher Levels:

When you add "At Higher Level" info you select a modifier to base this scaling on. You will be making 3 for each modifier. The structure is:

Add A Higher Level >> Scaling Level Value: (level) >> Modifier to Scale: (primary or secondary) >> Scale Effect: Additional Points >> (damage) >> Save Changes

(level) will be the character level the damage changes. You will be making one for 5th level, 11th level and 17th level for each of the damage modifiers.
(damage) will be how you want the damage to change by putting in the roll info or the fixed value. Note, however, this will be "in addition to the base modifier". If you progress by d8s then for  5th level put 1d8, 11th put 2d8 and 17th put 3d8.

And you're done.

Wow thanks so much for taking the time to do that! The big part I was missing was the "As Part Of Weapon Attack". Now I see the optional damage in the description of my weapon.

I can’t find the ‘restriction’ part of the secondary damage.

Quote from i_am_coffeelock_stay_away >>

I can’t find the ‘restriction’ part of the secondary damage.

 

 That's the "details" text box.