Getting the textbox.Text from another Form

I have a class. Inside this class I have a method. In order to perform a calculation, I need the value from another Form. The method does not get called by the code behind of the other Form....so I can't send it as a parameter.
In my method, I am trying a code line like this
TextBox my_add_city = (TextBox)MyShoppingCart.FindControl("add_city");
The Form name is: MyShoppingCart.aspx
The TextBox ID=txt_add_city
In my method I am trying to get the Text found in "MyShoppingCart.txt_add_city.Text"
Right now, I have a red squidily line under MyShoppingCart and when you hoover the mouse over it the tooltip says "The name MyShoppingCart does not exist in the current context".
Thanks, John Feeney
 
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