How do I take the value of text box in specific page in the main window frame

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C#

WPF

I create an wpf window in this window I add frame and button that was out the frame
In this frame I put source page which contain text box
when I click the button it must take he text that is in the text box and show it in message box
I try

Request_Pages.PageEnterValue ev = new Request_Pages.PageEnterValue();
res = ev.tbValue.Text;
MessageBox.Show(res);


Request_Pages.PageEnterValue :is the page where the text box is
tbValue is the text box name

when I clicked the button the message box is empty