The text of the Benn-Burt bill, otherwise known as the European Union (Withdrawal) (No.6) Bill , can be found here. There’s nothing in it that I can see that prevents Boris Johnson making demands as part of the extension request. The relevant subsection is this:
The Prime Minister must seek to obtain from the European Council an extension of the period under Article 50(3) of the Treaty on European Union ending at 11.00pm on 31 October 2019 by sending to the President of the European Council a letter in the form set out in the Schedule to this Act requesting an extension of that period to 11.00pm on 31 January 2020.
And the text of the letter is this:
“Dear Mr President,
The UK Parliament has passed the European Union (Withdrawal) (No. 2) Act 2019. Its provisions now require Her Majesty’s Government to seek an extension of the period provided under Article 50(3) of the Treaty on European Union, including as applied by Article 106a of the Euratom Treaty, currently due to expire at 11.00pm GMT on 31 October 2019, until 11.00pm GMT on 31 January 2020.
I am writing therefore to inform the European Council that the United Kingdom is seeking a further extension to the period provided under Article 50(3) of the Treaty on European Union, including as applied by Article 106a of the Euratom Treaty. The United Kingdom proposes that this period should end at 11.00pm GMT on 31 January 2020. If the parties are able to ratify before this date, the Government proposes that the period should be terminated early.
Prime Minister of the United Kingdom of Great Britain and Northern Ireland”
What here prevents Johnson from also demanding that Portugal becomes a British territory, with all the Portugese people required to leave by Christmas, as part of the extension request?