After some time of experimenting I dont think it is possible to use itms-services with credentials. I ended up passing the credentials directly in the url with:
https://Username:Password@plisturl
In the plist I also added the credentials to the ipa URL. I don´t think it is a clean solution to add the credentials in clear text but it is the only solution I see at the moment.
Keep me updated if someone has a better solution.