Button_Click method - Determine what the mouse button was pressed.

Try using the MouseDown event instead of the Clicked event of the button:

Example:

       method-id button1_MouseDown final private.
       procedure division using by value sender as object e as type System.Windows.Forms.MouseEventArgs.
           if e::Button = type MouseButtons::Left
              set textBox1::Text to "left"
           else
              if e::Button = type MouseButtons::Right
                 set textBox1::Text to "right"
              end-if
           end-if
           
       end method.

Thank you very much, Mr. Chris.

It´s easy, when you know.