How can I make it so that when an item is in the Hotbar, it gives the player an effect?

You can do the procedure to check the hotbar slots and if the item exists give an effect. I'm not on my PC right now, if you don't or others don't, I'll send you a screenshot of what it looks like.

0 through 8 are the hotbar slots

you then wrap it up it or statements so it checks them all

http://www.mediafire.com/file/q3tteqemuwu539j/itembonus.ptpl/file

Rather than having to copy and paste the same thing 9 times like Kinyoshi suggests, you can actually just use a loop to check all the slots. Print statements added for clarity (they are not needed to work).

The way I demonstrate below also stops checking other slots if the item is already found, so that the game doesn't spend extra time looking