Substitution of values in ASP .NET TextBox

Hi ASP .NET Gurus,

I have met non-standart situation (as I saw it). The problem is following:

I have TextBox on WebForm and the button near it. I need a peace of code which gives users possibility to make substitution of values in ASP .NET TextBox from other WebForm.

For example: User enters new Customer Account and needs substitute a Customer Group. So, he clicks on "Browse" button and gets to new page where we have Customer Groups grid. He selects one group by clicking on it and get back to entering of Customer Account (all entered info should be save), and the value of selected Customer Group substituted to indicated TextBox.

Hope I explain everything correct. :)