I want to add a custom field that is a combo box used for priorities?

[John Reasor]

I want the combo box to be set to the issue number in the project plus 1. If the number is selected to a place above the current position it will be inserted to that priority location and the remaining issue at and below the new location are adjusted down the priority list.

Please point me in the right direction to create this custom field.

[Nic Brough -Adaptavist-]

That is not priority changes, that is "ranking", and you'd need to write quite a lot of code to implement it safely and cleanly.  JIRA doesn't have a field to do that, but JIRA Software implements it.

[John Reasor]

Please explain this statement "JIRA doesn't have a field to do that, but JIRA Software implements it." Are you saying JIRA can or can't do this?