How to call code behind before javascript

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C#

ASP.NET

Javascript

I have 2 pages
page 1 contains an asp button which should call a code behind function then open the page2 as a popup window
on page2 there is also an asp button which should call a function from its own code behind and another function from page1 code behind then close the popup window

in other words: I need to transfer data from page1 to page2, and prepare another data in page2 before closing and inform page1 about closing to receive the data

note: I want to do this without user permission for the popup, so registering scripts from code behind? not what I am looking for!

"I use sessions to transfer data"

What I have tried:

I tried to call an invisible asp buttons click event using javascript
It works in page1 but not always

"page1 using master page"
"page2 webform"

At this code I used

document.getElementById('ContentPlaceHolder1_HiddenButton').click();

in page1 to create the data session before opening the popup. and used

document.getElementById('HiddenButton').click();

in page2 to prepare another data session for return. also used

opener.document.getElementById('ContentPlaceHolder1_HiddenButton1').click();

in page2 to inform page1 about new data session and popup closing.

page1 code

<script type="text/jscript">
     
       function OpenPopup() {
         document.getElementById('ContentPlaceHolder1_HiddenButton').click();
         window.open("WebForm1.aspx", "List", "scrollbars=yes,resizable=no,width=1000,height=600");
            return false;
        }

    </script>

 <asp:Button  ID="HiddenButton1" runat="server" Text="Button"  style="display:none"	
        OnClick="HiddenButton1_Click"/>
    <asp:Button  ID="HiddenButton" runat="server" Text="Button" style="display:none"	OnClick="HiddenButton_Click"/>

<asp:Button ID="btnOffer" runat="server" Text="Add Offer"  onclientclick="javascript:return OpenPopup();"/>

page2 code

<script type="text/javascript"> 
   function returnValues() {
    
            document.getElementById('HiddenButton').click();
           opener.document.getElementById('ContentPlaceHolder1_HiddenButton1').click();
            window.close();
     
    }
    </script>

 <asp:Button  ID="HiddenButton" runat="server" Text="Button" style="display:none" OnClick="HiddenButton_Click"/>

<asp:Button ID="btnFinish" runat="server"  OnClientClick="javascript:return returnValues()" Text="finish and close" />