You need to create a HMI tag to hold the page number and map it to a PLC tag. Then open the graphic in the editor and right click on it to open the Display Settings window for that page.Click on the Behavior tab. On top is the Startup tab. What ever commands you enter in the box will be executed when the display is opened. You can enter the command &Set HMI_Page 10. Where &set is the command, HMI_Page is the tag and 10 will be your page number.Now when this page is opened the tag will be set to ten. Replace the tag name and number with your values. You could also create a macro to do the same thing and put in in the Startup tab.