Connect. then open a web page. Should take you to the portal page.
NOW.. that said. I know one campground that puts Wi-Fi Passwords on the park info/map sheet... but the router is open, no password needed.
My Chrome and androids ask me to go to the log in portal and open it for me but my Widders box is not that smart.
Another "Trick" I have discovered.. one CG that uses that portal system I am unable to connect the housue since the router has no browser and it times out before I can get everything set up and browse to the portal.
My Phone has Wi-Fi Sharing. I connect IT to the park, and the house to it. Works.
Where I am now I connect house to park and phone to house also works.