How do I tell my parents I want to get a diagnosis for High Functioning Depression or Major Depressive Disorder??

There are lots of ways to go about doing this.

If it were me and I was scared of their reaction, I would do two things beforehand. 1) Research a good website or two that I could show them and explain why you think that the symptoms those websites describe fit you. 2) Look up a doctor or two that you want to consider going to. That way you have a plan and they don't have to think through what the next step would look like.

Sending you strength and peace. You deserve that. Don't let your brain talk you into doing nothing. @cyanTurtle8500