To be diagnosed with ms, do you have to be symptomatic?

Yes but: In general .yes You would have to have some symptoms, but some are subtle or not recognized by a patient but could be recognized by a good Neurologist

Not really: Many patients are diagnosed based on a characteristic MRI pattern, called radiologically isolated syndrome, which eventually can evolve into clinically definite MS. If you had a prior relapse which has now resolved, yet, MRI is abnormal, you might be classified as clinically isolated syndrome, which becomes clinically definite over a brief time.