I think a simple statement saying that you would like to have a morning cortisol level checked might work better than asking. I don't think you need to be overly assertive and you might add a "please" at the end of the statement.

Be prepared to answer questions about why you would like a morning cortisol level checked. There are many valid reasons for checking a morning cortisol level but some endocrinologists will say it isn't necessary if you don't have symptoms of adrenal insufficiency. In that case you focus on how difficult it has been to taper off Prednisone and why you wish to be off Prednisone.