An endorsement can be as little as saying "we should take the idea of extraterrestrial influence seriously", or similar remarks. Bill Nelson's remarks (

https://thehill.com/opinion/international/579303-nasa-chief-bill-nelson-latest-official-to-suggest-ufos-have/

It's not sufficient to say "extraterrestrial intelligence may exist" in a context divorced from UFO considerations, nor is it sufficient by itself to say "we don't know what UFOs are".

For the purposes of this market, a "high-ranking U.S. official" is any of:

(I'm open to persuasion if someone thinks I missed someone obvious. I'm deliberately only considering the executive branch, though.)

The statement must either be public, or made public (e.g. leaked, and confirmed by an independent source) by the end of the year. If leaked, the leak must come from two independent sources, and 

 sources must come forward before the end of the year.