window.onload wont execute in wordpress

Well don’t mix PHP and Javascript without the proper tags

<script type="text/javascript">
    jQuery(document).ready(function( $ ){

        if(location.href != "https://linestar.ca/contact-us/") {
            alert("Make sure you use the right contact Tab!");
        } else {
            alert("LineStar Welcomes You!");
        }

    });
</script>

As a sidenote, your website has lots of issues that you have to work out eventually.