Hi,
in the fsm on each buttons that receives the UGUI / ON CLICK you forward it by using the action "Send Event", that action will fill automatically the "sent By GameObject", and so this is a read only variable, setup by PlayMaker. so you can get it in the "Get Event info".
to actually SET event data, use "Set Event data", but in your case you don't need to, this sill be populated automatically as you want the sender, not custom data.
Bye,
Jean