Within the Creat-Account option,
you can choose between 6 diffrent domains.

I've got my own server, that's why i wanted to create a user on that one.

I did notice that it's possible if you do it like this:
username:
username@myserver.tld
server/domain: whatever

that way chatsecure 'ignores' whatsever selected within the domain list
and creates the user on my server (myserver.td)

-> Add some sort of 'other' to the dropdown menu, where you can enter your own domain name :)