Notify User When a Relationship is Added when the Source ItemType and Related ItemType have matching properties

I am trying to notify the user when a relationship is added where the Source Item has a property of _part_number that is equal to the Related Item property item_number, but continue with the action of adding. Basically just give a warning.

I have tried server side, got the message to be sent with an error message, but that stops the progress of the action.

I understand now that it needs to be a client side action, but I am having issues with getting the Items in the method to get the properties to then compare.

Any help with this would be greatly appreciated!