Need to know whether Home Office Reference number is given in email or send as hard copy to our address

Hi,

I have lost my BRP card and need to apply for new one. In the application I need to fill Home Office reference number. I could not find it in the letter I received with my BRP card. I read that it is given to us in letters. Can you please guide if it is send to us in hard copy or in email. If email, than please advise the subject line of the email.

Any help is highly appreciated.

Thank you
Radhika