cosign

Etymology

co- + sign

verb

1. To sign a document jointly with another person, sometimes as an endorsement.
2. (informal, transitive) To agree with or endorse.
   [Ilhan] Omar was left twisting in the wind earlier this year after facing spurious charges of antisemitism, a display of Democratic cowardice co-signed by Chelsea Clinton, Chuck Schumer, and most every other Democrat with a congressional leadership position. 2019-7-19, Noah Kulwin, “Democrats Fail the Left, Once Again”, in Jewish Currents

noun

1. The promotion of one musical artist (usually less successful) by another.
   Also, you can't base your entire career on cosigns and big features. At some point, your music has to stand on its own. 2013, Jesse Atkinson, The Independent Music Grind
   Big pump, don't stunt when it's showtime / Kinda funny how I didn't need a cosign 2017, Stormzy, Swifta Beater (lyrics and music), “Cold” (track 15), in Gang Signs & Prayer, performed by Stormzy