I was wondering if there is a way u can add potion effects to a custom item?

(Not a ContentTweaker dev.) You can make use of at least vanilla potion effects by using the performEffect(IEntity) method from IPotionEffect. You get an IPotionEffect from an IPotion, which in turn you get from the potion bracket handler. See IPotionEffect in the documentation.

Can u show how that script would be written I cant figure out how one would write it.