How will I become member of RuneScape forum?

No, Mylot is a PTC site, which is used to help put cash into your paypal account. The only problem now for many who used this source is that they require a verified account on paypal to pay for Runescape membership, hence, most people quit Mylot.

But to answer your question, no, to go onto Runescape Forums you must become a member of Runescape.

When you first posted a discussion or response in the Runescape forum it should have asked you if you wanted to add it to your interests after you put in ( or skipped ) the tagging screen. If you closed it out instead then you've been missing that screen. Watch for it next time you post to that discussion topic.