If all you want , however is just a new window that DOESNT close the existing form window, and know .js or dont want to risk it ( some people have .js turned off in which case your page would simply not work) .. you can just add target="_blank" to you link, this will open THAT SPECIFIC LINK in a new tab or window.
You could also just do the .js inline (ugh)
I assumed that the numbers given were a width and a height respectively and you wanted no status bar on the popup and
<a href="http://www.websiteslink/" onclick='window.open( "http://www.websiteslink/", "myWindow", "status = 0, height = 600, width = 635, resizable = 0" ); return false'>Terms</a>
BTW keep in mind that whether they leave the page or you open a new wind they are not still in the form , so the "read and agreed" check box still has to be checked in the original form ( but you probably already knew that)