You mean +use or something? If so, it's probably just "bind f +use". Although, I might have the command wrong (the bind part is right, the "+use" part might not be - check what HL2 has bound to E and use that).
Just be aware it's completely useless in most all TF2 maps, except like one or two.
Where do you ever "Use" things? You just shoot at it or whack it with your melee stick. --- i herd you like tanks so i strapped some tanks to your tank so you can blow up tanks while you smash - Weeza_Mon