Minecraft modding

[_Riley_]

So i know this isn't really coding since I'm using a tool to help me but this was the only section that fit into. I'm creating a mod using the program mcreator and it's an amulet that gives a potion effect from another mod. It's just for a server me and a few friends are playing on. When I go into the code section of mcreator and replace the vanilla potion effect with a modded one it gives me an error because it doesn't know what the effect is. So I'm basically asking how to do this.

 

On the other side I tried since this is a one item mod that I could maybe do just a simple /testfor @p {Inventory:[{id:"minecraft:(whatever item we choose)"}]} but the only way I can see that work is having a command block for each player and their specific ign to receive the portion effect. If there's an easy way to do this with command blocks that would be way more ideal I just don't know how. Thank you!