Green’s Dictionary of Slang

1. (US Und.) an endorsement, a recommendation; also as v., to praise or recommend for one’s own purposes.

1914		Jackson & Hellyer Vocab. Criminal Sl. 74: ‘With the proper send in I can twist this boob. Rib it up.’ Also used as a verb, to laud, to praise, with an ulterior motive.
1954		J. O’Hara Sweet and Sour 123: I was instrumental in getting a young man a job. I have done that fairly frequently. I have given a young man or young woman the send-in just as I was given the send-in when I was bucking cub reporter.

2. (US Und.) information about a possible crime.

1935

D. Lamson We Who Are About to Die 192: He gets a send-in on a soft touch up north [...] an’ he chases up there to look it over.