Werden is Hilfsverb in this case. And the tricky thing with zu? In most cases Verbs which are normally followed by an infinitive have this zu. Das Baby hat aufgehört zu schreien. Ich habe sie gebeten zu gehen. You have given a very special example. The sentence is a subject cláuse. It has the role of the subject in the main clause, although you did note write the main clause. Verbs in subjects clause normally carry the zu with the infinitive, if the subject clause follows the main clause. Es ist verboten Alkohol zu trinken. If The subject clause comes first you can dispense with zu. Alkohol trinken ist verboten, or, Alkohol zu trinken ist verboten.

The zu is part of the um ... zu "in order to" construction.

Wir kommen, um zu singen. "We come in order to sing."

Wir kommen, um unterhalten zu werden. "We come in order to be entertained."