Those trying to use the Surinder Singh route to be in the UK, cannot use the online system and must request a paper form. You need to apply by the deadline too as this route is under the EU rules and is not covered in the Withdrawal Agreement (if there is one). You can either phone and ask for this form or apply online and then they will ask for all your details, name, address, Tel phone no, DoB, citizenship, passport number, then they will send a form. Once completed, that form and all the documents will need to be sent by post.
Just as you would have needed to be granted the EU's PR, you will still need to prove your Centre of Life was in that EEA county and that the British citizen was/still is, a "qualified person": worker, or student/self suffciient with CSIs for everybody, etc, as per the EEA Regulations.