Wordpress Fetch API Admin Ajax 400 Errors - ajax

Question

When I submit my form to my wp-admin/admin-ajax.php I get a 400 error using fetch.
I have tried removing content type.
Hard coding the Fetch URL
using FormData
let formData = new FormData(form);
Using URLSearchParams
using them both
const data = new URLSearchParams(new FormData(form));
I cant get it to work and am out of answers online so thought I'd ask here.
Can you see an error in my code?
<form >
<input type="text" name="fname">
<input type="text" name="email">
<input type="hidden" name="action" value="add_user_details_hook">
<button type="submit">Submit</button>
</form>
<script>
let form = document.forms[0];
const data = new URLSearchParams(new FormData(form));
form.addEventListener('submit', function(e){
e.preventDefault();
fetch('/wp-admin/admin-ajax.php', {
method: 'post',
body: data
})
ADMIN-AJAX.php
function ajax_add_user_details_func(){
var_dump('hey!');
}
add_action('wp_ajax_add_user_details_hook', 'ajax_add_user_details_func');
add_action('wp_ajax_nopriv_add_user_details_hook', 'ajax_add_user_details_func');
Edit I do believe it has something to do with empty FormData as even when I iterate over the object with forloop the fields are empty:
Example:
for (var [key, value] of formData.entries()) {
console.log(key, value);
}
Returns no values