It appears this may be a duplicate thread of -> https://forum.dvdfab.cn/forum/software-support-english/youtube-to-mp3-youtube-video-downloader/410098-log-into-paromount-account-with-amazon-channel-sub which contains a screen shot.
As @twhiting9275 said, StreamFab relies on you setting up your streaming service accounts, and/or linking them (if possible) before you use StreamFab to download. StreamFab can not create, link or verify accounts, and it only passes your credentials to the streaming services you are trying to authenticate with.
Keep in mind too, that StreamFab's browsing component might not be a fully featured browser, so I wouldn't try using it as such as it might fail. I would sign up to streaming services, and do all that stuff in your normal Chrome, Edge, Safari or Fire Fox browser.
I know of two ways to subscribe to Paramount+. One is directly on their web site, establishing a user ID, password, and entering a payment method. The Other is subscribing through Amazon Prime Channels, where Amazon handles the billing.
If you subscribed to Paramount+ via the Paramount+ web site you can not link it to Amazon and download Paramount+ content with StreamFab's Amazon Downloader. You must use StreamFab's Paramount+ Downloader.
If you subscribed to Paramount+ through Amazon Prime Channels, you can download via StreamFab's Amazon downloader. Also you have the option to link it to a Paramount+ log in, then use the Paramount+ App/Web site and/or use StreamFab's Paramount+ Downloader. To link accounts, see https://help.paramountplus.com/s/art...on-Prime-Video I suggest linking accounts in your regular browser and NOT inside StreamFab's browser window. After the accounts are linked, then you can make use of StreamFab's downloaders that you have a license for.
I too subscribe to Paramount+ through Amazon Prime Channels. (And I have followed the Paramount+ Help link to link my accounts).
I have posted months ago, in another thread, that since I am subscribed through Amazon, downloading Paramount+ content using StreamFab's Amazon Downloader, I get better quality videos than when using the Parmaount+ downloader. Please see -> https://forum.dvdfab.cn/forum/software-support-english/youtube-to-mp3-youtube-video-downloader/403385-amazon-downloads-too-large#post403408 or https://forum.dvdfab.cn/forum/software-support-english/youtube-to-mp3-youtube-video-downloader/403269-amazon-disney-hack-possible?p=403347#post403347