I have the same issue, but trying to link themes to child epics.

I have to go to the epic, and do the linking to the parent theme from the child.

Doesn't make much sense not to be able to do the opposite, and it also doesn't look it should be that hard to fix.

I say fix, because i think this should not be a feature request, but a bug, or at least a known issue.

I disagree with this being a feature request and not a bug. This is heavily impacting for teams that are working across projects (most of ours do), and should be a basic functionality within Jira. We've recently (this month) migrated to Cloud and this is something I received complaints about from Day 1 after the migration. 

I understand the need to avoid the impact of a site-wide search from the moment the user starts typing, but in that case restrict the search to start after the project key is input, then limit the search to that project key, instead of immediately searching within the current project from the get go. 

The alternative for me is that if you click on the Add Child button at the top of the issue, you're presented with a popup that, in the same way as the issue create screen, has a project field at the start of the screen and then displays options to add after that. You could even have a checkbox to "show resolved issues" that by default isn't checked to restrict the search further from the start. (EDIT - Just realised this is pretty much what 751a2c764a27 suggested, so upvote for them, they're on to a winner, let's make it happen )

rchristian@atlassian.com The resolution suggested is almost same as 

1. Add an Epic in Project A
2. Add a Story in Project B
3. Link Parent in the newly added Story.

The suggested method is just a vice versa of above only

It would be nice if you can get the Create issue
"popup" open when someone click the "Add Child issue button" to select the desired Project

Please let us know when are you going to implement this?

Hey all,

Thank you for letting us know about this challenge you're having with our experience.

The way this functionality works is currently intended in order to resolve performance constraints that result from searching across all system Epics in this context. For this reason we will update this ticket to be a feature suggestion from a bug.

That said, while this behaviour expected, we recognise it’s far from a desirable experience and want to minimise the impact of it. We are investigating an improvement that would work within our performance limitations.
The improvement would allow you to directly input an issue key from the Child issue panel which would allow you to set child issues from different projects.

I recognise this isn’t the ideal method, but it should at least make it possible to add child issues without context switching.

06dacf0511be - Do you think this solution would be an improvement for you?

Cheers,
Rhys | Product Management, Jira Plans