display an intermediate screen in the transition from screen 1 to screen 2

How to display an intermediate screen in the transition from screen 1 to screen 2

Screen 2 contains a table with a database that takes some time to display. Switching between screen 1 and screen 2 The software disappears until Screen 2 opens. How can I post a message to the user "Please wait ..."

this my code:

this.Hide();
Form C = new Main();
C.ShowDialog();
this.Show();

i work on C# , WinForm

thanks