This is more complex that you think.
In order to do this, you must have access to both form instances: not the definitions, but the actual instance of the form that is running, and holds the information you want.
Using your names:
Assuming that your Button "button1" is on "formb", then if "formmail" contains the instance of "forma" then you can do it.
The first thing to check is: are you copying the right way? Is the text you want to copy in the TextBox "txtto" and the place you want to put it "txtAto" on the other form?
Put a breakpoint on the copy instruction, and check.
It is not a good idea to try accessing the content of other form controls: it ties the two forms too tightly together. You can't change one without checking is doesn't affect the other.
Instead, use a property:
Public Property ToAddress() As String
tbToAddress.Text = value
And access that instead.