These both Html Web Controls are embedded in to asp:Content i.e We are using Master Page. Now we are going to Add/Insert the JavaScript at Page_Load event of our Page.

 Button1.Attributes.Add("onclick", @"CopiedTxt = document.getElementById('" 
+ TextBox1.UniqueID +
@"').value; window.clipboardData.setData('text',CopiedTxt);");

Page.ClientScript.RegisterStartupScript(this.GetType(),"myjavascript1" ,
@"function PasteIt(){
CopiedTxt = document.getElementById('" + TextBox1.UniqueID + @"').value; window.clipboardData.setData('text',CopiedTxt);
}",true);

ClientScript is a new Object introduce in ASP .NET 2.0. Before ASP.NET 2.0, you would have needed to employ the RegisterStartupScript and the RegisterClientScriptBlock methods. These methods are now considered obsolete. Both of these possibilities for registering scripts in ASP.NET 1.x required a key/script set of parameters. Because two separate methods were involved, there was an extreme possibility that some key name collisions would occur. The Page.ClientScript property is meant to bring all the script registrations under one umbrella, making your code less error prone.