I am trying to contact a potential supervisor about doing a master’s by research. I have already wrote my email by I am unsure about what to write for the subject box of the email. Different people have advised me on different things and I am unsure what do. Does anyone know what is a good subject line?
It's an email subject line. Don't over-think it. Any variation on "Master's by research" would be perfectly fine.
When I was applying for PhD, I used "prospective graduate student" as a subject line, which was quite sufficient. If your potential supervisor accepts students for different programs, you can specify the degree you are seeking (Master's) in the subject line.
The rest of the information (your background, interests, etc.) should be in the email body.
Good luck :)
This is the email header I used when reaching out to professors last year (adjust for your use accordingly):
Enquiry about potential PhD opportunities for Sept 20XX
I also agree with what others have said. In addition, make sure to check the department/supervisor website to see if you can glean any information about whether or not they are actively recruiting graduate students. While these notices are not always accurate, sometimes professors explicitly say they are/are not seeking new students.