Just as the title claims I'm trying to put in a magic item on here. That adds a specific type of attack. Here is the word description of the item given to me by it's creator.

"This cestus allows you to make unarmed attacks as 1d4+STR mod that deal either Piercing or Bludgeoning, and have a +1 attack and damage modifier. While attuned the gauntlet grants you Cold Resistance. As part of an attack you may choose to deal an additional 1d6 Cold damage on a successful hit, this removes the cold resistance, which recharges at the end of your next turn."

Obviously some of this is up to me being aware of the awkward recharge rate, but adding the unarmed attack seems to override my normal UAs which is fine as well, I just have to be aware. But I'm wondering if the same will have to be true for the cold damage? Or is there a way to make this item that I'm not thinking of?

What exactly are you trying to do? I don’t think I understand what you are asking.

There is no way to create an action using a magic item.

Whoever gets the magic item will need to create the custom action themselves on the sheet.

I am trying to add a weapon attack for DnDBeyond's character sheet action page for the item. The creator of it is very specific in that it's not a weapon, which is helpful since I'm a monk. Essentially it's Brass Knuckles issue. Unarmed weapons aren't a thing in 5e so DnDBeyond doesn't have a thing for them. I think unstuck with just making my own custom action on my character sheet.