Best way to display pop up message box in C# ASP.NET with string in center of browser

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C#

ASP.NET

Hi Experts,

I have to display pop up message in c# asp.net. I am using below code for displaying messages. But in some browsers this pop up is appearing top of browser. The page becomes ugly. So I want to display messages using any script that displays center of screen with model.
If there are any articles about it. Please refer me links. or Give me some hint how to get this in own way.
Right now i am using below code to display pop messages.

ScriptManager.RegisterStartupScript(this, this.GetType(), "ALTER", "alert('" + objTrans.ErrorMessage + "');", true);

ScriptManager.RegisterStartupScript(this, this.GetType(), "ALTER", "alert('Record Approved Successfully'); window.location = 'RvlsAppMnu.aspx'; ", true);

What I have tried:

<pre>ScriptManager.RegisterStartupScript(this, this.GetType(), "ALTER", "alert('" + objTrans.ErrorMessage + "');", true);

ScriptManager.RegisterStartupScript(this, this.GetType(), "ALTER", "alert('Record Approved Successfully'); window.location = 'RvlsAppMnu.aspx'; ", true);