To add a new user custom field:
Log in as a Jira admin
Navigate to Jira Settings > Apps > PROJECT TEAM > Profile Custom Fields page
Click the Add Custom Field button
Select the Type of the new field
Enter a Name for the new custom field
Optionally, type a Description
If you choose to add a Multi Line Text field, you can add one or more items that represent the possible values of the field.
Choose where the new field is to be displayed. If you check Show on User Advanced Profile, it will be displayed on the User Advanced Profile page of every user. If you check Show on View Issue Screen, the field will be displayed in the "User additional attributes" section of the View screen of the Jira issues, but only if the field has a value.
The User Advanced Profile page displays all the fields, regardless of their Show on User Advanced Profile setting, to the users that have permissions for editing the field values.
Click Save