Ok, I have found various ways to do it.

Using the Page_Load() could be such as:

ShowText.Attributes.Add("onclick", "javascript:ShowAlert(window.document.Form1.FirstName.value)")

Or, generating the entire HTML on the Click() event of the control such as:

Response.Write("<SCRIPT>alert(""This is my text."")</SCRIPT>")

But, I am mostly interested in a more direct way to achieve that directly from the Click() event such as:

MyObjectForJavascript.Something="ShowAlert(""This is my text."")"

Is that possible?