In my experience, best to practice on the various U-XX leagues before trying S-League in order to get to know the match rythm. These matches usually are held in either schools or stadiums and usually no one will bother as long look like you belong there.
If you dont mind cropping the image, 70-200mm is adequate. If its a daytime match, can use a 1.4x/2x TC to get even more reach at the sacrifice of some of the iq.
Can also shoot away matches especially in M`sia as usually they dont care whether you use pro gear or handphone camera. Its an opportunity if you want to try your hand at photographing the game at professional level.
How to get accreditation is by looking through media sources that cover the S-League as there is a chance they might be looking for a sports photographer. It`d help if you have experience which is where photographing the other leagues will come in handy as you can use your best images for a portfolio to submit to the editor.