jQuery - Ajax post request loads same page with form data added to URL

I think you should use <input type='button'> instead <button> and just use .click() event of that <input type='button'> as below.

Change

<button id="myButton">Register</button>

To

<input type='button' id='btnRegeister' value='Register'/>

And in JS

Change

$("#registerForm").submit(function()

To

$("#btnRegeister").click(function()

So your entire code become as below now ..

<form id="registerForm">
            <input type="text" pattern="[A-Za-z]{1,20}" placeholder="First Name" name="guestFName" title="Up to 20 alphabetical characters" required>
            <input type="text" pattern="[A-Za-z]{1,20}" placeholder="Last Name" name="guestLName" title="Up to 20 alphabetical characters" required>
            <input type="email" placeholder="Email" name="guestEmail" title="Must be a valid email address" required>
            <input type="text" pattern="08[36579]-\d{7}" placeholder="Phone Number" name="guestPhone" title="Must be an irish mobile number of format 08?-7 digits" required>
            <input type="password" pattern="(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{8,20}" placeholder="Password" name="guestPassword" title="Must be 8 or more characters long and contain at least one number and one uppercase letter" required>
            <br>
            <input type='button' id='btnRegeister' value='Register'/>
</form>

$(document).ready(function(){
    $("#btnRegeister").click(function() 
    {
       $.ajax(
       {
           url: "URL_GOES_HERE",
           data: $('#registerForm').serialize(),
           type: 'POST',
           async: false
       })
       .done(function(response) 
       {
           console.log(response);

           var result = JSON.parse(response);      
       })
   });
});

Try Above code, it is working for me

EDIT: You should include the 'type' attribute to your HTML:

<button type="submit" name="submit"></button>

Using the 'preventDefault' function will prevent the default form submit / redirect- This will stop the unintended behavior

$("#registerForm").submit(function(e) 
{
    e.preventDefault(); // Add this

    $.ajax(
    {
        url: "URL_GOES_HERE",
        data: $(this).serialize(),
        type: 'POST',
        async: false
    })
    .done(function(response) 
    {
        console.log(response);
        var result = JSON.parse(response);      
    })
});