Post JSON with javascript

Question

I want to post my register form to my rest api using JSON and JS, can any1 help me with this, or has some advise with this.

I also just recently started coding in HTML, JS so try to explain it as easy as possible.

And yes I have researched some things but I can't understand them, and also visited other topics regarding this but I don't get the hang of it.

My JS

window.validateForm=function(e) {
e.preventDefault();

alert(document.forms["registerform"]);
var x = document.forms["registerform"]["password"].value;
var y = document.forms["registerform"]["password2"].value;
if (y != x) {
    alert("The passwords do not match. Please try again!");
    return false;
}
console.log("Fired");
return true;

};

My HTML so far:

<div class="form">
<form name="registerform" method="post" onsubmit="return validateForm()">
    <label>
        <span>First name:</span>
        <input type="text" required="required" name="reg-firstname">
    </label>
    <span>Last name:</span>
    <input type="text" required="required" name="reg-lastname"><br>
    Email address:<br>
    <input type="email" required="required" name="reg-email"><br>
    Password:<br>
    <input type="password" required="required" name="reg-password"><br>
    Retype Password:<br>
    <input type="password" required="required" name="reg-password2"><br>
    <!--<label>-->
        <!--<span>Phone-number: </span>-->
        <!--<input type="tel" required="required"-->
               <!--name="reg-telnummer"><br>-->
    <!--</label>-->
    <!--Mobile-number: <br>-->
    <!--<input type="tel" required="required"-->
           <!--name="reg-mobielnummer"><br>-->
    <br>
    <input type="submit"  value="Submit"><br>
</form>

</div>