To make a TextBox automatically select its text we can have a GotFocus event handler as below:
In GotFocus event of the Textbox we can have 2-3 lines of codes to make it auto select when it gets the focus. The code is shown below:

Text1.SelStart = 0
Text1.SelLength = Len(Text1.Text)
OR
SendKeys "{home}+{end}"