Using Bootstrap, this is JQuery that will open a modal if a specific line of text is found within the URL.

OpenModal_JQuery.js

//Insert JQuery URL into header. Below is from Google.
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>

//Place below code in body of page. It will open the modal by whatever ID is there when the page is ready.
$(document).ready(function(){
    if(window.location.href.indexOf("sometexthere") > -1) {
        $("#id").modal();
    }
});