Okay, I believe you are saying you want to have the issue custom form data populated onto the custom form of the project when you are converting.
To do this, the fields need to be on the project form in addition to the issue form—then the system will know to map the fields. You have to use the same fields for it to map them, so either use the same form or make sure the fields are aligned.
This video shows you how the fields will map if you use the same forms.
https://experienceleague.adobe.com/en/docs/workfront-learn/tutorials-workfront/manage-work/issues-requests/create-a-project-from-a-request