Depends on the browser, but if you're using Safari, under one of the menus (I forget which off the top of my head) you'll see a "Disable pop-ups" option or something very similar to that, with a check mark next to it. Select it, and if you check the menu again, the check mark is gone.
HOWEVER, a lot of sites say to disable popup blockers just as a kind of cover your **** sort of thing. By and large it's really not necessary, because after Firefox came up with the idea of only blocking popups not initiated by the user (like by clicking on a link), shortly after everyone else copied the idea. So, I would try the site with the popup blocker enabled, see how things go. If it doesn't work, then you can go and disable it and try again. I say this in large part because IIRC, with Safari the popup blocker is either on or off, there's no whitelisting like you can get with Firefox and Chrome. So you have to remember to turn it back on again every time you turn it off for some site. Now maybe my info is out of date, as I haven't used Safari for more than maybe 10 minutes in a stretch for years. So there may well have been improvements I'm not aware of, and I would expect someone to come along and say as much shortly if that is the case.
If you're using some other browser, you'll have to name names.
I need to turn off my "popup blocker" before I can enter a website. I can't seem to find it using Spotlight. And what is the reason for it anyway? I thought it might be used to trap unwanted junk, but I'm trying to get to a site to retrieve my user name or password. Geeze Louise...........
Thanks, Old Granny Lola