I've only known how to use the authorization URL to have a user connect their account to my website. Is there a way to do this authorization via an HTML form? For instance, the user inputs their Google email and password on my website and it authenticates that account without them ever having to sign in and out of accounts on Google and then visit the auth URL.
The reason for this is to connect multiple Google accounts to one account on my website. It would be a huge pain to ask them to log in and out of Google for every single account they want to attach. It'd be a lot better for the user experience to just type the info in a form on my site and have it authenticate. Is this possible in PHP or in any language?
This is a bad idea, do you really want to be responsible for user's Google password? Do you think users will trust you with that information? I know I wouldn't and I'd be highly suspicious that your sight is a phishing scheme with that behavior implemented.
Use OAuth 2.0 or OpenID instead and save yourself the headache of dealing with user passwords.