The Jira Service Desk project's field layout is powered by Jira's screen configurations, which include screens, screen schemes, and issue type screen schemes. For the customer portal, Jira Service Desk provides a simplified version of the screen used by the Create Issue operation to keep the user experience smooth. This is why, for you to be able to add a field to a request type, the field must first be added to the Create Issue operation screen:

For the agent's view, Jira Service Desk makes full use of Jira's screen and field management features, so you can set up different screens for the Edit and View Issue operations.