I created a simple sample based on your requirement. In the sample, the asp.net Button is placed inside WebDialogBox content. When the button is clicked, it will set the value of ShowOnLoad property to “false”.

protected void Button1_Click(object sender, EventArgs e)
{
    dlgBox.ShowOnLoad = false;
}

Please let us know if you have different scenario.

End sub

please help me

One of the default behaviors of WebDialogBox is the TrueModal behavior. The modal behavior means that when DialogBox shown, the parent window which owns the DialogBox should become unclickable.

If your WebButton is located in the parent window, then you need to set WebDialogBox to use Modeless mode, instead of Modal mode which is set by default. When the WebDialogBox is using Modeless mode, the background object will no longer be blocked. This allows you to interact with the Webpage while the WebDialogBox is active.

To use modeless mode, simply set IsModal to false.

I modify the sample so that now it has two buttons. Each of the buttons is used to show and hide WebDialogBox. Please have the sample tested on your end and let me know whether it helps or not.

I sent a picture to explain better

There is a javascript method that you can use to close WebDialogBox, it is "CloseDialog()" method. Below is the snippet code.

function Button1_onclick() {
    var dlgBox = ISGetObject("dlgBox");
    dlgBox.CloseDialog();
}