you can disclose before hand and get an existing condition covered on your insurance.
but if you dont want it covered, or you know they wont cover it, or you think they will charge too much to add the extra cover, you dont need to disclose beforehand.
instead, when you make a medical related claim, they will ask for authority to contact your doctor for your records. then their own medical people analyze the claim vs your medical report to determine if the claim is related to an existing condition or not.
did the forms n stuff for a claim after i had what they thought was viral pneumonia during the 2018-19 season. there's funny story about cough suppressant drugs containing dxm in there too

turns out its also a party drug.