send in for something
and send off for somethingto dispatch an order for something to a company or other body making a public offer of goods. I sent in for a new product that is supposed to make my hair grow back. Did you send in for that country-and-western CD as you said you would? I sent off for the proper contest entry forms.
send someone in for someone
to send someone into a game as a replacement for someone else. The coach sent Jill in for Alice, who was beginning to tire. Ted sent Bill in for Watty.