Javascript form validation with password confirming

I am trying to write a registration page and I am having the hardest time.

<form  action="insert.php" method="post">
    <h1>Registration:</h1>
    <input type="text" name="first" placeholder="First name">
    <input type="text" name="last" placeholder="Last name"><br />
    <input type="text" name="username" placeholder="Username"> <br />
    <input type="password" name="password" placeholder="password"> <br />
    <input type="password" name="confirmPass" placeholder="Confirm Password"> <br />
    <input type="submit" value="Register">
</form>

This is my code on the registration page. How do I call my JavaScript function and have it still post and do the action? I made this as a simple module which is separate but I would want to merge them

<input id="pass1" type="password" placeholder="Password" style="border-radius:7px; border:2px solid #dadada;" /> <br />
<input id="pass2" type="password" placeholder="Confirm Password" style="border-radius:7px; border:2px solid #dadada;"/> <br />

<script>
    function myFunction() {
        var pass1 = document.getElementById("pass1").value;
        var pass2 = document.getElementById("pass2").value;
        if (pass1 != pass2) {
            //alert("Passwords Do not match");
            document.getElementById("pass1").style.borderColor = "#E34234";
            document.getElementById("pass2").style.borderColor = "#E34234";
        }
        else {
            alert("Passwords Match!!!");
        }
    }
</script>

<button type="button" onclick="myFunction()">Sumbit</button>

I do not need help merging them, I know I would have to switch some things around, but if they are together, how do I call the JavaScript function?

This will be done more neatly after I figure out what to do, So what would be the best way to call