When creating a workitem by the blue "Create" button the appearing dialogue shows a warning about the missing option to enter data if the the workitem has a form included. Only a switch to the portal let the agent enter all data. If continued in the internal agent´s view the workitem needs to be created first and after that the form data can be added additionally. 

The switch to the portal OR the two step data entry in the internal view is bad UX and should avoided. Ideally the form is within the "Create" dialogue (maybe even by expanding the form first before an agent is able to enter the data).