Add TrueStrike to a weapon under "actions" tab

There is no way to do that currently in the character builder.

The best you can do is create a custom action for it, though this will only be able to get the damage roll right for some weapons (because the dice roller buttons can't do multiple types of dice at the same time). What I've been doing is a custom action for the attack roll and then just rolling the damage with physical dice.

I also have simply made a secondary custom action and named it for example "True Strike Repeating Shot" or whatever (since I play artificers) for every weapon to ensure the attack roll is correct. And then I have a separate cheat sheet reference that reminds me I can add extra True Strike dice if that's true for the character's level. 

You do have the option of, after your attack, navigating to spells and then clicking the damage for True Strike after, but you do have to remember.

For something like a dagger, you'll still need the regular attack for opportunity attacks, which won't use True Strike. 

Quote from SunIsGettingRealLow96589 >>

I swear a year or so ago, there was something on dndbeyond that let me select what ability to use for attack/damage. 

Well thats dissapointing

There are specific options built in for certain things that change the attack ability, like Pact of the Blade and the Battle Smith Artificer feature. But I don't think it's actually possible to do that correctly for True Strike because of the way the damage roll works.