Hmm I'm a bit confused about what you're trying to do. Blockly is a library that allows other applications (like AppInventor, MakeCode, Scratch, etc) to include blocks-based coding. Are you trying to build an application that allows /other people/ to build login forms? Or are you trying to build a login yourself.
If you're trying to allow /other people/ to create login forms I recommend starting with the
getting started codelab. It's a great tutorial that walks you through all of the steps of setting up a basic blockly application. Then you can check out the
guides section of the docs, specifically the
creating custom blocks document. This provides more detail about creating your own blocks than the codelab. You can also check out the
blockly developer tools, which allow you to define your own blocks with blocks!
However, if you're trying to build a login form yourself, I don't think Blockly is going to be much help :/ But there are lots of tutorials on the internet that should help you out, or you could use a website builder that includes built-in login support =)