how to insert c # server code into javascript event definition?

It is impossible to do this through markup, only in the code-behind. Therefore, remove onblur in the markup and create a handler for Init 'and control:

 <asp:TextBox runat="server" ID="tbFirstName" onfocus="$.ClearInput(this);" OnInit="tbFirstName_Init" /> 

And in the code:

 protected void tbFirstName_Init(object sender, EventArgs e) { ((TextBox)sender).Attributes["onblur"] = string.Format("$.SetInputText(this, '{0}')", regFirstName); } 

Or, if you can C # 6, then:

 protected void tbFirstName_Init(object sender, EventArgs e) { ((TextBox)sender).Attributes["onblur"] = $"$.SetInputText(this, '{regFirstName}')"; }