To elaborate, you have to go into XGUI, find ULX settings, MOTD settings, check the boxes 'show motd on player join' and the box 'show from a url instead of motd.txt" and place the URL in the box.
You can't just replace everything in the file with a URL.
You should also be able to use console, from server or superadmin game console,
ulx showMotd "http://www.google.com/"
(using google as example, would be your url)