To fill out login forms, you have two options: use the MyGlue Chrome Extension, which was designed for this, or open the MyGlue web app, go to the Passwords list view, and copy and paste your credentials into the webform.
The browser extension detects when you go directly to a site's sign-in page, and lets you choose whether to sign in using a stored credential or not.
When you first get to the sign-in page, the MyGlue logo will display in the password field. If you want to sign in, you click on the logo, which prompts you to choose a stored credential to fill in to the username and password fields.
The end result is your username and password are inserted into the webform for you, as shown in the example below.
The password field is detected automatically. But depending on the layout of the fields, the extension may not put the username in the right field. If this happens, you can change where the username is filled: