Occasionally, 1Password does not offer the Masked Email dialog on web pages. You could create addresses directly via the Fastmail settings, but then the additional integration functionality won’t be available. Instead, you can now use this handy bookmarklet to force the full Masked Email experience on any webpage.
If you are viewing this on github.com: the first step is required to be done from the website instead.
Add the link below into your browser favorites via drag and drop, right click or long tap on mobile. If your browser does not allow bookmarking links directly you can also create a bookmark of this page, copy the link below and manually paste the content into the bookmark.