The User Settings page allows users to select a default repository for a Jira project.
Go to Jira User Profile ➜ Git Integration: User Settings and you will be seeing the following screen:
If your Jira instance have multiple projects, use the search bar to look for the project or several projects (using the search criteria) to work on and assign a default repository for each project.
On the Repository column, default repositories can be set using the dropdown list. Utilize the search function in it to return specific repositories according to the search name.
This configuration affects the Create Branch, Create Pull Request and Create Merge Request dialogs.