Get opportunities of XOR Gateway using REST

I have the following situation:

What i want to do is to get a dropdown menu in my Angular application where all opportunities of books are listed. So i can choose between ‘Harry Potter’, ‘Moby Dick’ and ‘Robinson Crusoe’. When i selected one book and press submit i send a POST /complete via REST.

The Dropdown should contain ‘Harry Potter’, ‘Moby Dick’ and ‘Robinson Crusoe’, whereby the options should always be the same like in the Camunda Model:

My questions here are: how can i get the information which books i can choose via REST? Is that even possible and is that how it should be done?

Thanks in advance