The sender address should be specified in the backend and will be sent to Transsmart to be used as the sender address for the booking that are created.
The sender address can be set via: stores > configuration > general > general > store information.
For manual bookings in the MyTranssmart portal, your business address is set as standard. This can be adjusted by logging into the MyTranssmart portal. Then go to 'Manage account' at the top and choose Dispatch locations. Here the sender address is set which is used for the bookings.