How to make this function in c#

If I understand you correctly, and you want to exit from LoginBtn_Clicked without having to check the return value of CheckPassword and 'returning' - well, you can't.

You could do what you originally had, or this:

void LoginBtn_Clicked()
{
   if (CheckPassword("xxx"))
   {
       // stuff in here
   }
}

Or, you could get CheckPassword to throw an exception, but that's an awful awful solution. Why do you want it to be void? It absolutely should return a bool, because the password check either fails or succeeds.

But why? The result returned by the CheckPassword method should branch the code into one of two directions. Change the method to not return a bool is not only pointless, but is also ill-advised, not to mention adding a certain level of ambiguity to the code.

If you want that the Function should return void then this is the method not a function beacause function always return a value.

void CheckPassword(string inputPass)
        {
            if (passWord != inputPass)
                {
                 //Your Code if password is correct
                }
            else
                {
                 //Your Code if Password is wrong
                }
        }

Call CheckPassword on click of Login button.

void btnLogin_Click()
       {
           CheckPassword("xxxxxxxx");
       }

You could just use the CheckPassword function to exit if not true, else the app can carry on like this

void CheckPass(string inputPass)
        {
            if (passWord != inputPass)
                Application.Exit;
        }

        void btnLogin_Click()
        {
            CheckPass("XXX");
            //Carry on if password correct
        }