General Discussion

Sanders did not wait until the primary concluded to back Platner. He explicitly endorsed the first-time candidate early on, leveraging his national progressive network to boost the oyster farmer’s campaign.

In fact, his support before the primary was fierce. For instance, when The New York Times published a pre-primary report detailing allegations of volatile and "toxic" behavior from Platner's ex-girlfriends... however, in spite of that, Sanders aggressively defended him.

Sanders insisted on social media that Platner was "the only candidate who will do something" to challenge a rigged economy and a corrupt campaign finance system (of course, with the usual buzz words and accusations) telling voters they needed to focus on the working families of Maine rather than personal attacks on Platner (IOW: ignore the facts).

He also acted as a political cover and shield for Platner against the "institutional Democratic Party". When national Democrats began encouraging mainstream figures like Maine Governor Janet Mills to enter the Senate primary race, Sanders publicly scolded the party establishment and warned them to stay out of the midterm primary, protecting Platner's path to a landslide victory.

So... I think this might provide some clues to answer to your question about who "recruited" Platner.