I think the way this was done was some WH staffer who knows Bernie called Bernie on the phone and told Bernie what a wonderful guy he is and how much everybody loves him and well, we need a favor...
There could have been a bribe thrown in to sweeten the pot a little, but flattery is usually the first tool in the toolkit.
Another “trick” they use is to call asking for policy advice (more flattery) before “closing the deal”.
Of course the “policy advice” will be laughed at and ignored after the phone call is done.
Bernie is anxious that they’ll take one of his big Lakeside houses away from him. Remember. They gave him houses so that he would drop out when he was ahead. What a goof.