I would offer you to use apex:detail as your solution.Below is the code example

    <apex:page standardController="Event">
   
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script>
    <script>
    $(document).ready(function(){
        ReadyFunc();
    });
    function ReadyFunc(){
        var a = $('[value$=pections]').first().attr('onclick');
        $('[value$=pections]').first().attr('onclick','ShowPopup();'+a);
    }
    
    <apex:detail relatedList="true" inlineEdit="true" relatedListHover="true" oncomplete="ReadyFunc();"/>
    </apex:page>

You can do anything you want inside ypur document ready function automatically.

more information about : [apex:detail][1]


  [1]: https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_detail.htm