Whether or not you can do this depends on the design of the web site. If you can determine the correct syntax, you may be able to encode the username and password as part of the URL that opens the web page.
But the syntax and even if it is possible will vary from web site to web site. And many web sites will not tell you anything about this part of their design for security reasons. (They don't want to make it easy for someone to use an app to crack their security through repeated log in attempts using commonly used passwords.)
So, basically if a simple login page contains only of two fields, there's no way to "insert" username and password other than crack the syntax of the URL?
Remember that you are interacting with a website controlled by who knows what applications configured in any number of different ways to control user access. Sometimes the website publishes documentation such as you find for Google Maps where you can determine the correct URL syntax and sometimes they do not.
And the only way you can send data, such as the account name/password, to that site is to make it part of the URL.