You can use IMessageFilter to trap WM_LBUTTONDOWN messages and then check to see if the cursor is within the TextBox...something like:
Public Class Form1
Private WithEvents filter As New MyFilter
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
TextBox1.Enabled = False
Application.AddMessageFilter(filter)
End Sub
Private Sub filter_LeftClick() Handles filter.LeftClick
Dim rc As Rectangle = TextBox1.RectangleToScreen(TextBox1.ClientRectangle)
If rc.Contains(Cursor.Position) AndAlso Not TextBox1.Enabled Then
TextBox1.Enabled = True
TextBox1.Focus()
End If
End Sub
Private Class MyFilter
Implements IMessageFilter
Public Event LeftClick()
Private Const WM_LBUTTONDOWN As Integer = &H201
Public Function PreFilterMessage(ByRef m As System.Windows.Forms.Message) As Boolean Implements System.Windows.Forms.IMessageFilter.PreFilterMessage
Select Case m.Msg
Case WM_LBUTTONDOWN
RaiseEvent LeftClick()
End Select
Return False
End Function
End Class
End Class