How to call class function in Windows Forms ?

In the case of the method you show, in order to call it you need an instance of the containing class: an example of the class that the method is part of, created at some point with the new keyword:

MyClass mc = new MyClass();
...
mc.dbConnection();

If you change it to a static method:

public static void dbConnection(string ProjectName112)
{
  string strProject = ProjectName112; //Enter your SQL server instance name
}

Then you don;t need the instance, you can just use the class name:

MyClass.dbConnection();

But then the method can't access any non-static field, properties or methods of the MyClass class.

But...the example you show does nothing useful: it returns no value, and the string variable strProject is destroyed when the method exits, so nothign will influence the outside world.

I think you need to think a bit about exactly what you are trying to achieve here: this probably isn't going to work...