I was told that it does not work in English. Which surprises me, because it seems like a perfectly logical construction...
Nope, it's a totally different function and it's called the same as in English, just with one 's': ambasador. Jestem ambasadorem.
On a separate note, notice that if you take only one letter from this sentence, you arrive at "Jestem osłem" (I am a donkey / I am a stupid person).
Though the Nominatives are "poseł" and "osioł", so not that similar.