Assuming you're talking about LH, isn't it these:
I could be wrong though but i'm not sure that will work for what you're trying to do. I think tireless march works because it's ends up being an enchantment on the unit that stays there. Actually thinking about it... that might work but maybe you would also have to put an ongoing effect onto the target also so that it knows the spell is active still.
Regardless I don't think you'll be able to cast the spell on a new unit, won't you have to cancel or whatever the active one first? Not sure how tireless march works I haven't tried it in LH.