I have a homebrew race that states Once per day ou can add your Str Modifier to the attack and damage roll of one dexterity based attack. You can't use this feature again until you finish a long rest. How would I set this up in the Homebrew?

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I would just make it a no action and write out the text. It's not too terrible to just add the number yourself once a day. With no specified attack and two stats in use, I'm just not sure how you could program this in.

I would make it a “General” Action with the “Special” Activation Type, and use something like the following snippet to tell the player how much to add: 

Once per day you can add {{modifier:str#unsigned}} to the attack and damage roll you make that uses Dexterity instead of Strength for the attack and damage rolls.

There’s no way to get the system to automatically add the number since temporary effects are not currently supported, but the snippet will at least present the correct number to make it easy for the player to do the arithmetic themselves.