Is there a way to create a login form on my website, without using an API, which would allow users to log in to another site such as Facebook or Twitter?

It is indeed possible and often quite easy to achieve. Simply specify the URL you want in the form's action attribute, and it's done. Unfortunately, this is also why phishing scams exist. Furthermore, it's not even necessary to create a website; you can accomplish the same thing using a local form in an HTML file. It's alarming to consider how often this method is employed for illicit purposes.

In my opinion, a regular password manager - whether integrated in a browser or an extension - simply requires a login web form with standard field names to function properly. The user need not be concerned about the order or visibility of the fields on the page, as long as they can be accessed through the DOM to be filled out and submitted.

Dynamic generation of fields from code is the biggest challenge for password managers. Therefore, it doesn't matter how the authors of the page choose to display the login form, as long as it's present somewhere on the page. In the worst-case scenario, you can create a separate web form and display it within a frame without leaving the active page.

When it comes to integrating login functionality with external platforms such as Facebook or Twitter, it's important to consider the following aspects in more detail.

1. Security and Authorization:
When users log in to a website using their social media credentials, the process involves authentication and authorization. Social media platforms require the use of their APIs to handle the authentication process securely. By using the official API, the platform can verify the user's identity and provide the necessary authorization to access their account.

Attempting to create a login form without utilizing the proper API for authentication may expose the user's credentials to potential security risks. It could also lead to unauthorized access and compromise the user's account privacy and security.

2. Legal and Terms of Service:
Social media platforms have specific terms of service that outline the requirements for integrating their authentication systems. Attempting to circumvent these requirements by creating a login form without using the official API could potentially violate the platform's terms of service or lead to legal implications.

3. User Experience and Trust:
By using the official login buttons or widgets provided by the social media platforms, you can offer users a familiar and trusted login experience. Users are accustomed to logging in with their social media accounts, and using the official methods ensures a seamless and reliable authentication process.

While it may be technically possible to create a login form without using the API for social media logins, it is not advisable from a technical, security, legal, and user experience perspective. It is essential to adhere to best practices by utilizing the official login methods provided by the platforms through their APIs to ensure security, compliance, and a positive user experience.

If you want to allow users to log in to your site using their existing accounts on other services, you should use a Single Sign-On (SSO) solution, such as OAuth or OpenID Connect. These protocols allow users to grant your app limited access to their account on another service, without sharing their login credentials. This approach is more secure, scalable, and user-friendly than attempting to create a login form that logs users in to another site without using an API. By using SSO, you can provide a seamless user experience while minimizing the risk of security breaches.