On a hit, the target must make a DC 13  Constitution saving throw or be poisoned for one minute.

I'm trying to recreate a weapon from a module, and I don't know how to make the above line happen properly.  I can of course just put the text in there and the player requests the roll every time, but I'd much prefer to do it properly as this will happen a lot and if it is properly baked into the weapon roll then it will arrive at my vtt with a roll request for me (the dm). Streamlining everything.

As a backup I'll just need to ask the player to roll from within the vtt but they (and me too tbh) all prefer using dndbeyond for character sheets.

Which weapon, which module? If you tell me I can look at it for you and see if it is actually coded at all.

To be honest, there’s no easy way to do that, and even if their were, it would just display the save DC on the wielder’s sheet and the target would still have to be verbally told and roll the save manually. There is nothing you can put onto a DDB character sheet that could possibly affect any other sheet or statblock. The official item is probably just text in the description, DDB doesn’t do stuff so it works elsewhere, it’s technically against site policy to port DDB content to a VTT. You must be using something like Beyond20, and it’s usually those 3rd-party extensions that do the porting and connecting.

You could make the poison as a spell (like a cantrip) and add it to the item with a fixed save DC. Just make a unique subrace for your spell so you can add it to that instead of any list you’re gonna use, and use that for all your “spells that aren’t spells.”