1. It is very important that you create a custom field (Workflow Group QD) within the appropriate request (not a task) screen set (aka entity type). If you wish to use the custom field between different screen sets you have to create it within the root screen set 'Request':
2. The screen set assigned to the workflow template request could be not less important:
Custom field created in a screen set 'MyScreen1' will be not visible in a request with assigned screen set 'MyScreen2'. But if you have created a custom field into the 'Request' screen set it does not matter what type is assigned to your request - field will be visible in every type of request.
3. To assign a task to a request field you have to select the field item named with prefix "Request No/" (e.g. "Request No/Your Custom QD Field"):
I hope this will help.
Gytis