The link issue between different JIRA instances is not available from the New JIRA Issue View.

To create one of these links, the users must either disable the New JIRA issue view or go to the detail view. They can't do this type of links from the boards for example. It only show internal projects issues, the application links aren't included (yet) in the New view.

Normal View link issue feature:

New JIRA Issue View link issue feature:

Could be useful to get this feature in the New view too and avoid moving to other screens just to create a link between issues from other JIRA instances