You will be asked to disclose your medical condition(s). My top tip is to have all that information to hand when you are applying for a quote so you can accurately answer all the questions.
Insurers do not ask to see your formal medical records when you apply for travel insurance, but may request them if you make a claim.
Another question that travel insurance companies have to ask and which you may find distressing to answer, is whether your medical condition is terminal. You might want to consider having a friend or relative with you while you do this or ask them to make the call for you.