How to decline a completion offer

I just changed the "fullopt" setting:

set completeopt = preview,menuone,longest

default:

set completeopt = preview,menuone

After a long use, as it was, I decided that I prefer the first entry to be selected by default.

I have one annoyance. I would like to try a complete one, if the match I am looking for is not listed, then drop the proposed completion and return to the original text. How it's done?

It seems that I remember from the distant past that such behavior exists but randomly scary buttons did not turn out to be fruitful and search engines do not work for me.

Thanks for any help.