It is just blood typing - just like you would test for your ABO blood type. I just told my doctor I wanted to have some blood typing and told her what I wanted. Don't even talk about the BTD.
Ask for ABO phenotyping, Rhesus phenotyping, Lewis phenotyping and MN phenotyping.
It looks like you already know your secretor status, so why do you want the Lewis phenotyping. I see you don't know whether you are positive or negative (Rhesus)? Or maybe you have just not included it in your avatar? If you do not know it, that would be the best approach. It is very common for people to know if they are positive or negative.
How do I convince my doctor (who is not familiar with the BTD) to give me a script for Lewis testing?
Doctors in my city are a bit difficult, and always need some sort of reason.
I would like to be prepared before I go.
The best thing to do is go to your local blood donor center and ak for the tests. They already know what they are and you can probably get them done cheaper than going through a hopital blood bank. You can also bypass a MD who is usually clueless as to what these tests are.
These tests may have already been performed anyway if you have been a regular blood donor.