I am trying to homebrew brass knuckles, but can’t change the base stats on the item for the life of me. When you homebrew a weapon you have to pick a base weapon to base it off of, but brass knuckles aren’t available in those options. What’s the closest I can get to get the stats I’m looking for? My Dm has them as a magical item, 1d6 + 2 base and an additional 1d6 force when attuned and activated. 

I'd probably say a club. It's a bludgeoning weapon whose damage is based on throwing a punch. 

Thank you!! I’ll try that 

Find a base weapon that matches the core damage die you need - in this case 1d6 bludgeoning.  I believe maces deal 1d6 bludgeoning.  Modify your item as needed from there, and simply ignore it when the automatic text suggests it's a mace.

Alternatively, represent the weapon with a custom action on your sheet and a placeholder item in your inventory. 

I’ll try that as well, thank you so much!!

For that additional 1d6 force damage when attuned and activated, there is not easy way to add additional damage dice to a weapon. You can add additional damage dice and types to an existing weapon, but it will not give the appropriate buttons on your character sheet for you to click on anything for them.

If you use the Beyond20 extension, it will pull those additional damage dice and roll them in a custom roll. But that is relying on something outside of dndbeyond to accomplish what you want and doesn't carry over if you don't use that extension or it breaks one day.

You can create a custom action (or a simple feat with an action) that adds the additional damage type to whichever action tab you want (similar to how sneak attack or favored foe work on the Other tab). All that takes some effort and patience to get right and even then it adds something outside of the actual weapon. It just gets complicated.

As for customizing other weapons, give this a quick read. 

Or just do what Yurei1453 said and use a mace and change the name to brass knuckles, since it has no other weapon properties and is already bludgeoning damage.

DDB doesn't allow the creation of new base weapon types, Mary.  The system has no options or tools for doing so.  You can plaintext a new base weapon all you like, but there's currently no way to implement a weapon that isn't written in a D&D book somewhere. 

Quote from MaryFlory >>

One option is to base the brass knuckles off of a handaxe or a mace, as they are both one-handed weapons that deal bludgeoning damage similar to the brass knuckles. You could then adjust the damage dice to match the 1d6 + 2 base damage and add the additional 1d6 force damage when attuned and activated. Another option is to create the brass knuckles as an entirely new weapon type, such as a "knuckle-duster." You could then use the existing weapon types as a guideline for balancing the weapon's stats and abilities.

 I would not use the handaxe and the damage would be listed as slashing and thrown. Brass knuckles are neither. Club or mace would be the best choice but you can't change the damage die on the club. 

I would personally go with the club as 1d6 for brass knuckles feels high. Clubs also have the benefit of being Light.

Quote from Wysperra >>

I would personally go with the club as 1d6 for brass knuckles feels high. Clubs also have the benefit of being Light.

I agree with this. Plus if you use the club as your base and keep the light weapon property, you can have two of them and there's your two-weapon fighting.

They really need to add a function to allow us make base weapons/items.

Quote from Xeresia >>

They really need to add a function to allow us make base weapons/items.

I only very rarely find myself needing to invent a weapon from scratch. 99% of the time there is something that is close enough to make it work.

If it isn't a weapon, then you can go ahead and make whatever you want =)