ASP textbox event when you click inside the textbox

I saw this example but this is using JavaScript

ASP TextBox JavaScript OnKeyup Event
https://forums.asp.net/t/1656156.aspx?onkeyup+in+asp+textbox

I want to use C# or VB

On a ASP.NET Web Forms page.

I have a TextBox that looks like this:

<asp:TextBox ID="TextBox1" runat="server" ></asp:TextBox>


When I click in the textbox with my mouse how do I save the text "you clicked in the textbox" to a ASP Hidden Field using C# or VB?