How do i get an item from right clicking a custom block with an custom item

Within the block tab, triggers, on block right clicked. Create a procedure:

(The action result part dictates the players arm swinging when right clicking the block)

If you want the original item to be retained then add it/them back to the inventory after and check for how many there were.

If  [item in main hand] = custom_item
      Set item in main hand to:
                       [custom_item_2]
      
      Return action result type PASS
      
Return action result type FAIL

If you search for the code I put up there you will find the procedure blocks that I am referring to.