Perhaps that seems obvious, but I receive many requests from want-to-be-published writers who ask for my endorsement. I refuse them.
Think of it this way. Suppose I spend hours reading a proposal and writing an endorsement and the book never gets published. That does seem unfair, doesn't it?
Also, those who ask for endorsements before their manuscripts are accepted believe that the words from a famous name will sell the proposal. That's not true. But even if it were, wouldn't you feel terrible to think that you sold your book only because a celebrity liked it?
Here's what you can do. If you know someone of influence, on your book proposal write something like this: I can reasonably expect an endorsement from . . . The endorsers don't have to be famous, only influential. The CEO of Amazon or the President of Yale carry influence, even if readers might not know their names.
As a professional writer,
I seek endorsements only after I've sold my manuscripts.