Get data from DB and show in customer portal when creating a ticket

Hello,
I am new in Jira development and need an advice for solving this problem:

I need to have a custom filed, which should be auto filled with Reporter’s manager’s username. The manager’s username is kept in other SQL DB. I need to show the custom filed with already filled in information before a ticket is created, so that a customer is able to change his/her manager’s username, in case the manager is on vacation.

Please help with a good advice to achieve this goal

Hi,
Will the user be able to fill in the manager’s username by typing it or by choosing the corresponding username as an option from a list of suggestions?