If subprojects is enabled from the add-on settings (enabled by default) a small subproject dialog trigger button is added next to normal project selection combobox on issue create dialog.
Subprojects overlay is opened just under the project field so that you can see current project selection. Clicking on normal project selection field, pressing 'ESC' key or clicking anywhere on the issue dialog will close subprojects overlay. You can also use 'Tab' key to navigate between filter text field and subprojects tree. Pressing 'Enter' key on a subproject or clicking on a subproject will select that subproject and close the overlay.
Some projects may not be available on subprojects selection overlay because your JIRA administrator may have not add it to subprojects hierarchy. If you don't see your projects, please ask JIRA administrator to add it to subprojects hierarchy.
