How do you add bonus damage to a D&D Beyond character sheet?  I have a drow rogue, and I want to add the 3d6 poison damage to her attacks, but can't figure out how to add the secondary damage type. 

I don't know what 3d6 poison damage you're talking about, but you could homebrew a magic item that deals X + 3d6 poison I believe.  Definitely you can homebrew an item that deals 3d6 poison damage.

 DNDBeyond does not currently support temporary/secondary bonus damage dice on magic items. The closest equivalent of what you want would be the Serpent's Fang longsword.

If you use Beyond20, then it will automatically recognize dice in the items description and let you roll them from the sheet, but otherwise, you'll need to make the poison its own feature, if you want it to be actionable.

I don't believe you can just add 3d6 poison damage to all your attacks. As I have tried to do this with my rogue. I had to make a copy of my weapons and add the 3d6 poison damage to that. 

So I have 2 weapons the original with no extra damage and a second one with the extra and I just use which ever one I need.

Thanks, @KeldonLighthand, I guess that makes sense, roll the primary attack, and then roll the two damage slots.

Thanks, @Memnosyne

In this case, the attacks aren't magic items, just trying to use drow poison on a scimitar.  The character is a drow soulknife rogue.

Drow poison doesn't deal damage.  It's a DC 13 Con save - on a failure you're poisoned for an hour, and if you fail by 5 or more, you're unconscious for the same hour or until you're damaged or shaken awake.

Thanks, @quidraco, I must've misread the description, will play it that way now.

I know this is an old thread, but what I do is I create common magic item which adds a modified Homebrew version of Booming Blade or Greenflame blade to the character sheet just make sure to delete all the higher level stuff, and the extra damage for moving or being a secondary target.
Then change the damage type to what you need, the number of dice, and the type of die.

This will then add the damage type at the bottom of any weapons that this effect applies to.

So you can then roll the weapon damage and then click on the bonus damage if it applies without tabbing back and forth through things.

it's a bit of work to streamline things. But I like to have all the stuff on one screen so I do not hold up others during my turns.

It would probably just be easier to create a custom action to do what you want to do. Benefit is you can modify it on the fly an you don't have to mess with the homebrew tool.