How To Place Textbox In ComboBox

I have this code inside am event in a combobox. Because I don't know the placement off the textbox before I have run the code in the combobox:
Dim txttotal As TextBox
txttotal = New TextBox
txttotal.Name = "txttotal"
txttotal.Location = New System.Drawing.Point(x4, y)
txttotal.Size = New System.Drawing.Size(100, 15)
txttotal.TextAlign = HorizontalAlignment.Right
Me.TabPage3.Controls.Add(txttotal)
I can see the textbox thats works fine. But in another event I will give txttotal a value
But get the error txttotal not declared.