I'm playing an artificer, and my DM is letting me homebrew my own firearm. He gave me the OK for the weapon's trait to use Intelligence instead of Dexterity in the damage roll. How should I represent this in DnDBeyond homebrew settings? I can't seem to get the damage roll using INT to show.

I mean - are you a battle master? I think they get to do that with no need of homebrew.

(That's the "Battle Smith" subclass. They can make attack rolls with a magic weapon that use INT for the modifier.)

The one thing you should know about the "Bonus - Magic Item Attack with Intelligence" modifier is that it will apply to all your equipped magic weapons, not just the firearm with the Bonus modifier.

Make a magical item that is a copy of a regular weapon. from there you can change which stats it uses.

hey, not to be rude, but the question was how, and this is probably the most smug and least helpful post in response to a question I think I have ever read on the internet. This is quite literally responding to the question of "how do I do this thing?" with "just do the thing". 

Quote from pjosephgorman >>

hey, not to be rude, but the question was how, and this is probably the most smug and least helpful post in response to a question I think I have ever read on the internet. This is quite literally responding to the question of "how do I do this thing?" with "just do the thing". 

 i think you are interpreting what i said wrong...
but thats literally how you do it, not just do it. i literally said "create a magical weapon that has a base of your weapon that you want, then just choose the stats you want to use."
its as easy to do as that. 

There is no way to designate which stat a weapon uses. The best you can do is give it a modifier that makes your character use a given stat for all magic weapons. Please see the Homebrew item & Equipment FAQ #10 for additional information: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).

but how do you choose the stat you want to use?

What do you mean? The system automatically defaults to the higher stat.

I recently ran into the same problem and from what I've seen there doesn't seem to be a clear cut way, which is kinda odd. If all else fades I will just give it a magic bonus that would make the to hit/dmg equal the stat Im wanting to represent. 

Click the Add a Modifier when making your custom item.

Select Bonus in the top left box

In the next box to the right, Click Magic Item Attack (stat you wish to have the weapon work with) 

make sure to select the stat again in the box to the right of it and that's worked for me, the only other issue I've run into is typically the weapon has to be one that is attuned to in order for this to work.

Hope this helps