Remember Vb.Net everthing is an object.
So you have to call the form and the textbox.
Code:
Module CodeModule
Public frmMainOne As System.Windows.Forms.Form
End Module
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
frmMainOne = Me
End Sub
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
'Gets the caption from the first form
txtBox6.Text = frmMainOne.Text
End Sub