Using a Msgbox open a form within a subform in navigation form

I have made a Navigation form named as "Navigation_Form". It has 3 navigation buttons ("NavBtn1", "NavBtn2", "NavBtn3"). In navigation buttons property "Navigate target name" i have assign forms name Form_1 for "Nav_Btn1", Form_2 for Nav_Btn2 accordingly. The problem is that i want if i press NavBtn3 it will show a msg "do u want to go to Form_1" by pressing Yes it navigate me to Nav_Btn1 and show me the Form1 as well, I totally confuse where to start and where to end