Once you have uploaded your fillable PDF form to your template, completed your mapping from the template to the PDF, enabled the DocuSign integration and added DocuSign signature elements to the PDF, you can now choose the provider of the PDF.
Click on Edit PDF details in the upper right hand corner
In the provider drop down select TD Ameritrade
Selecting TD Ameritrade as the provider, will enable Knowledge Based Authentication (KBA). Once all signers are done, you should forward the completed document to TD Ameritrade along with any required supporting documents.