Finally, I was able to connect with the ServiceDesk API, making an Atlassian Account to the customer withouth permissions, using Basic Auth. I cannot use OAuth2 because the Atlassian authorization page doesn’t allow him to log in, so it’s a quite good workaround until OAuth2 is implemented on Jira API.
Now, I’m trying to get the same information I can get from Jira, but that’s for another thread.
Thanks for your help.