When requesting coverage for a shipment, you can choose to start an auction.

To change which carriers are part of the auction, click on the drop down menu under "Carriers included in auction."

You can choose from all carriers in your network; if this route is one of your designated lanes, can choose from primary and backup carriers on that lane, only primary carriers on that lane, or only backup carriers on that lane.

To change the primary and backup carriers on a lane, go to the "Lanes" page of your Shipper App, for details, see this article.