The Benn Act specifies the exact text that the PM is mandated to send:
http://www.legislation.gov.uk/ukpga/2019/26/schedule/enacted
The PM is given absolutely no discretion.
I see the form to be used. Boris could openly say “we insist on conditions X, Y, and Z and any extension would be pointless if the EU is not prepared to accept those conditions because we will never agree to present to Parliament any agreement with the EU that contained those terms”.
Then go ahead and send that form letter off. Now the bell is in the Yurps’ court.......remember A veto by even one member state to a further extension results in a clean break Brexit. Granting a further extension by the EU will be seen as accepting the conditions Boris laid out.