I would like to have a custom field that is like a label (allows creation of extra results when typing), but only allows one to be chosen.
For example, I would like to have a field "Source" with no values initially. Once a person creates an issue in JIRA, and gets to the "Source" field, they would be able to search for possible values, but since none will exist initially there would be a button to create a new entry (see screenshot below).

I would use labels, however the problem with labels is that the user can add multiple results and I only want one result chosen per JIRA issue.
I am open to the usage of add-ons but have checked many different custom field add-ons (nFeed, premium custom fields, etc..) and I have not figured out how to do this yet.