It means you would have to inform the council office that you got married to a British citizen. They want your legal papers for marriage to register, so far you are not appeared in the exam so must submit your documents in town council office and get registered, while applying for CSS exam, must mention this thing in your application/form. That's it. You are eligible so nothing to worry about. Rest, contact fpsc, they will better guide you in this case.

Be Blessed.