Landmark Global doesn't support specifying a Sender's Address when booking shipment's via Shiptheory. This is a limitation of Landmark Global's API.
Whilst Shiptheory show's a Sender's address in your shipment details, all shipments booked with Landmark Global will automatically use the Sender's address associated with your Landmark Global account itself.
If you need to make any changes, please contact Landmark Global directly to do so.
For any other help or if you have any other questions, please get in touch with the Shiptheory Support team