Allow Default Screen be used when creating new request instead of Screen associated with Create

Customer requested that when creating an issue within Service Desk customer portal, he would like to have a specific Field A shown, however when creating an issue within the same project, he would like the field to be hidden.

Our service desk request type A is mapped to our issue type B. Due to its screen scheme, issue type B has a "Default" screen and a screen for the "Create Issue" operation.
Field C is availble in the "Default" screen but is not removed from the "Create Issue" screen. Field "C" might, for example, be the field "Fix Version/s", which is not necessary when creating a new issue.
However, I want field C to be visible for the customers inside the Sevice Desk customer portal. Therefore I want to add field C inside the Service Desk settings (Service Desk > Request type > Request type A > Edit fields > Add a field). But field C does not appear inside the selection. Apparently, only the fields configured for the "Create issue" screen appear here, even though the field selection for the request type is used for all operations inside the customer portal.
Is this working as intended or a bug? If it is w.a.i., what is a possible workaround to not see field C when creating issue type B inside JIRA but to see field C inside the customer portal?

He would like the customer portal not to follow the JIRA specified screen scheme.