Please consider a more direct way to create/edit custom fields when creating/editing request forms.
PROBLEM: Currently, there is not a way to pick a custom field as a part of building your request form questions and it's not the most intuitive use to know to go build the custom field, make sure it's linked to the correct folder/space, make sure your request form is linked to the correct folder/space, and then set up the mapping.
SOLUTION SUGGESTION: When creating/editing request forms,
1) there's an option to create a question that will link to an existing custom field - you can then choose from the existing custom fields available to you, it'll populate the options in the request form and you can review for accuracy and finish building your form.
2) you have the ability to edit the custom field you choose is also available if you need to add new options or update the names of those options.
3) you can decide that you want a question to map to a new custom field and create the custom field directly from the request form area so you do not have to leave the form, build the field, come back to the form and then map to the field. Instead, you can do all of that directly from the form editor.