onfocus="this.select()"
onclick="this.select(0,this.value.length)"
<asp:TextBox ID="TextBox1" runat="server" CssClass="textBoxClass"
onfocus="this.select()" TextMode="MultiLine" ReadOnly="True"></asp:TextBox>
<asp:TextBox ID="TextBox1" runat="server" CssClass="textBoxClass"
onclick="this.select(0,this.value.length)" TextMode="MultiLine" ReadOnly="True"></asp:TextBox>
<input type="text" onfocus="this.select();" value="TextMessage" />