You have the easy option and an advanced option. Your easy option is that you already have a variable "forum_name" that has your forum name, so why not pass this on to your new thread page.

People click view lounge and it goes to the page view.aspx?name=viewlounge and you query that paramter am I correct?

Then from that page you call /newthread.aspx?name=viewlounge and just on your page_load event, just fill the textbox in with that parameter.