If i send my resume via email, and its to the HR meaning im not applying for any specific position, what should the email of my subject be?

not all companies are like that, as I know, media prima and astro do have to option to deposit the resume to HR, HR will then decide which department to place you. Example this is from astro:

Found nothing suitable or matches to your qualification and experience? Fret not, you can email us your latest CV/Resume at future@astro.com.my