When a sprint has started, it's only possible to add in existing product backlog items to the sprint backlog.
However, Scrum states that the sprint backlog can be added to directly by the development team at any time, as they often need to elaborate on existing product backlog items, adding in requirements, tasks, etc.
The only way to currently do this is to add an item to the product backlog then move it to the sprint backlog.
This doesn't follow how it should work when using Scrum. Not all tickets need to exist in the product backlog first. It's quite longwinded having to workaround this, Any plans to change?
Add item directly to active sprint backlog
No 'add item' button available. Instead have to add to product backlog then reassign to current sprint. Not compatible with Scrum work patterns.
I guess it's just a matter of preference. The functionality is there, just not the exact way you are asking for it. The existing functionality using the Create button doesn't add the additional step you described.
Personally, if I'm creating a new issue that I want to go in the current sprint, I'd prefer to use the create button in the navigation bar at the top of Jira, because I probably have other details I want to enter besides the name of the issue. The create button allows me to put in all the details and specify that it goes into the current sprint. It's also present on any screen, so I can use it directly from the board and don't have to be on the backlog page. I can also use it from the backlog page, so it's not really different from the create issue link you are asking to see for an in-progress sprint.
Not having the Create issue link on the backlog page under an active sprint prevents you from unknowingly, accidentally adding an issue to your current sprint.