If Workflow Policies have been created by your Autotask administrator, you must ensure that additional fields (either standard Autotask fields or custom-created fields) that are marked as required are only required in the application, and not in the Web services API.
If you do not do this, Autotask service tickets cannot be created through the OnPlus Portal.
To make sure that fields marked as required through Admin Workflow Policies are only required by the application, go to Admin > Service Desk > Dashboard > Workflow Policies and verify that any policies for required fields are specified as Only required in Autotask application. Here are two examples:
Require Issue and Sub-Issue Type fields for service desk tickets must be set to Only required in Autotask application.
Require Work Type Name field for service desk tickets must be set to Only required in Autotask application.