In the form that contains the Textbox, you want to do something like
this:
public string CustomerName
{
get { return this.customerNameTextBox.Text; }
}
then from the other form you can say:
form1.CustomerName
to get the value.
(Of course, change the name of the property, etc., to something more
meaningful for your program.)
BTW, I disagree with Nicholas's suggestion. You most likely _don't_
want to expose the entire Textbox via a property when all you want is
the contents, and those contents likely have some semantic meaning (as
in they're a customer name, an invoice number, an address, or something
like that). Exposing the entire Textbox reveals _how_ your form is
displaying / receiving information. All you want is the information.