Currently Forms field cannot be linked to user picker custom fields in team-managed project.
Allow for user picker custom fields in team managed projects to be shown as a Linked Jira Field.

Workaround:

The global custom field created in Jira is supported under Forms of Team-managed business project.

Steps to be followed:

1. Navigate to Jira settings -> Issues -> custom fields.
2. Create a user picker field.
3. Navigate to  Team-managed business project settings-> Issue types -> drag and drop the user picker field created -> Save changes.
4. Navigate to Froms.
5. Drag and drop the User picker field.
6. Save the changes.