Inventory Slot Reference

[Renothian2]

Anyone know what the XML command would be in an XML file to add to a switch/button the ability to activate/use my horse whistle which is NOT equipped, but in a certain inventory slot all the time?

In short, a command to activate/use whatever is in "n" inventory slot number that I can add to a button. Sorry, I am not into UI dev. so not about to read pages of instructions, just need help with this one thing. Thanks.