Yes you can do that. I do with opnsense. The username and passwd are not obvious though - they're probably not what you use to login to the ISP portal with.
Most ISPs will have a brief FAQ on how to use third party equipment with the basics of what settings are important for your connection. You just need to enter them in to pfsense correctly. Also, sometimes searching for "<ISP_name> pfsense" can find useful blogs and articles.