Before "The Authentic Life of Billy The Kid" was performed Thursday night at the Kennedy Center's Terrace Theater, author Lee Blessing, as winner of the American College Theatre Festival play award, was given his Dramatists' Guild card, the promise of a Samuel French edition of his play, a $2,500 check and a contract with the William Morris agency.

He now, undoubtedly, will be invited to a vast suite of offices in New York, where promising show business careers are screened and certified, to receive some hard-nosed, paternal counsel about his future. The drift of the counsel ought to run like this:

"Lee you are a talented writer and we are honored to have you with the agency. That scene, for instance, where old Billy the Kid - if he is Billy the Kid - and old Pat Garrett try to settle just how many people Billy ever really killed, 6 or 21? You had the audience hanging on the edges of their seats in that scene. That actor - Brruce Shapiro? - he was a help, of course, the way he played the Kid like some crazy hamster with the D.T.s. We should have signed him up, too.

"But not to beat around the bush, Lee, there are plays meant to be seen and there are plays meant to win prizes. Your play was a helluva piece of work - from the prize-winning aspect. What could be more American and at the same time, less risky than Billy the Kid and Pat Garrett? A terrific choice! And Myth versus Reality - what a theme! Heavy, but not too heavy.

"Those lines you gave the journalist. They just tied it all up like a Christmas present. 'The West is what you make it and that's all it is.' And his telling Pat, 'You didn't kill the boy, but you killed his name. That's more important.'

"Everyone was bowled over by the eerie production. That Twlilight Zone set. Those ghostly, swinging saloon doors. That rocking chair that rocked without anybody sitting in it. The wind howling through the rafters."Some people, it's true, might call the whole thing a little pretentious, Lee, especially all those reenacted shoot-outs toward the finish. But the proof's in the pudding, we say. You won the prize, and here you are, and there's nothing to stop you from trying something a little lighter, a little peppier the next time out. Is there?

"For our part, we look forward - and we mean this sincerely - to collecting a healthy share of the profits from the rest of your life."