I'm creating a weapon that on hit drains 1d4 con. I know how to add +1d4 slashing damage to the weapon, but not how to add "+1d4 con". Any thoughts on this?

It's probably not a supported effect since 5E doesn't have rules for ability score damage or drain the way older editions did.

Yeah, that will just have to be in the description, there is no modifier for that.

Really? That sucks. Makes no sense either. It would just be extra text in a database (slashing, piercing, .... Con, Str, Wis, etc). Plus, there are DND 5e creatures that drain stats on attacks. A Shadow does it (Strength Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 2) necrotic damage, and the target's Strength score is reduced by 1d4.) That would be a perfect ability to mimic on a weapon

Does DnDBeyond allow to just add random text (I'm new to this tool. I've created stuff in HeroLabs, like Mystic v3)

I’m unfamiliar with HeroLabs. I tell ya what, find a magic weapon that does the same/similar thing, “create” a magic item based off of that one, and see how DDB did it.

I'm googling items that do that ability. The best i can find is items that cast spells, but not drain items.

Quote from Mossad76 >>

I'm googling items that do that ability. The best i can find is items that cast spells, but not drain items.

 Which is precisely why that modifier does not currently exist. Now make a copy of the Shadow to see how they did it for the monster, and copy that.

Tried that before I posted the shadow idea. They don't have it listed in the attacks other than in text

Which is what I told you to do in the first place.

I fully understand how to write a description and wasn't asking for support on how to write a description

I'm looking to have it displayed in the weapon stat line. So instead of it showing "1d4 Slashing" it shows "1d4 Con Damage" Putting in the description requires that I have to click open the item and read it...which means it might be forgotten in the management of a battle scene.

You know

Wrath's Revenge 5 ft. reach +12 2d6+14 +1d4 Con Martial, Heavy, Two-Handed

If D&D Beyond doesn't support the mechanic of inflicting ability score damage, and all evidence suggests that it doesn't, then the answer is going to be the same regardless of how you ask the question.

What I have patiently been trying to explain to you for the past two days is that what you want is impossible to do.

In one way I want to give you the benefit of the doubt that you were just trying to help, but each post I reads from you has snark oozing out of it. Your final comment solidified that thought.

At first I was legitimately telling you exactly how to do what you wanted to do. I was in fact being helpful. You expressed refusal to believe me. I gave you advice on how to research the solution for yourself. You came back and said that didn’t work. I gave you another piece of advice on how to research the answer. (After all, “Give a man a fish, feed him for a day. Teach him how to fish, feed him for a lifetime.”) You said that you had already tried that and it worked the way my initial suggestion worked. After 3 days of trying to help you, just for you to finally learn that I was legitimately solving your problem for you in the first place.

I spent three days helping you. I could have just ignored you after I first solved your problem and you didn’t believe me. There are hundreds of thousands of users on DDB, how many of them took three days to help you? Oh, that’s right, just the one. Me. And now you want to blame me for “Snarking” at you after being the only person on a planet of 7,000,000,000 people who actually took the time to help you. Guess I just won’t bother next time.