Count Rupert and Ed1960 are right.
Just to be clear, if you are new, it goes into your inventory, which you can access via the Spells Inventory icon at the bottom of the city page. Once you click on it, click on the "Instants" tab. Regiment instants are grouped last (at least on PC - mobile may be different). The token will stay there until you click on it, but save it until you need it. Once you click on it, the squads will be added to your Army Camp. You can see your army camp by clicking on your Barracks building and selecting the "Army Camp" tab.