Corrected me if im wrong, you want to show the option that you selected in the select list option customfield (A) in a textbox customfield (B) ? If yes, you will need a plugin to achieve this like scriptrunner.
Hope this helps.
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events