how to get the text box value of one form to another form

There are several ways of doing this, depending on your circumstances in what you are trying to achieve. If you want to pass the username as a string from the login form to the main form, you can create a constructor in your main form that takes a string as a parameter, and then pass the username as the parameter. You could also create a public property in your login form that returns a string, and access that from other forms. Basically, the best option will depend on what you are trying to do.

You can use "LoginName" Contron from our Login Toolbox available in Visual Studio.
Alternatively, You can also use State Management for this purpus.