I'm afraid it is not possible to associate tickets between two JIRA applications under different domains using JIRA Cloud. To create this type of link, it would be necessary to configure fully reciprocal between your JIRA site and the other JIRA site containing the issue(s) you want to link to, and that's not allowed in Cloud.
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG