Is it possible to get redirected to the external URL once the order is submitted on the pressero B2B site ?

Answer:  Unfortunately it is not possible to redirect to some external website page once the order is submitted in Pressero as with the current behaviour it goes to the Order Summary Page.
As an alternative you can add the External Link(clickable link or a button using HTML editor) in the "Checkout Successful Instructions page" under Admin > Site > Site Name > Settings > Instructions Tab.
By doing so once the order is submitted in Pressero they will be normally redirected to order summary/confirmation page however they will see some kind of custom instruction added on the confirmation page something like below:
Order has been submitted, please visit "External link URL".