FETCH JavaScript PHP process the data on the server sent with fetch

I have this doubt of how to process the data brought from my form, I am using javascript with a fetch to receive the data of the form, but I have the doubt of how I should process them in php, nor is the click event of the send button working, the problem is that the server seems not to be receiving the array sent from javascript with the data, agradesco if you give me any source to delve into the topic of fetch api, I am new to javascript and php

my Javascript

registrar.addEventListener("click", () => {
    fetch("../add.php", {
        method: "post",
        body: new FormData(frm) //frm es el id del formulario
    }).then(response => response.text()).then
        (response => {
            // console.log(response);
            // si la respuesta sel servidor es "ok" arroja una alerta personalizada
            if (response == "ok") {
                Swal.fire({
                    icon: 'success',
                    title: 'Registrado',
                    showConfirmButton: false,
                    timer: 1500
                })
                frm.reset();
            }
        }
        )
})
  <form action="" method="post" id="frm">
                      <div class="form-group">              
                       <br>      

                    <div class="form-group">
                        <div class="form-group">
                          <input type="text" name="name_usu" id="name_usu" class="form-control form-control-md" placeholder="Nombre completo" required >
                        </div>
                        <input type="text" name="phone_usu" id="phone_usu" class="form-control form-control-md" placeholder="Numero de teléfono" required>
                      </div>

                        <input type="email" name="nom_usu" id="nom_usu" class="form-control form-control-md" placeholder="Email" required></div>
                        <input type="text" name='torreApto' id="Torre_apto" class="form-control form-control-md" placeholder="Torre y apartamento" required>
                      <label for="FormControlSelect1" class="text-light">Escoja tipo de residente</label>
                      <select class="form-control" name="sel_apto" id="sel_apto" required>
                        <option selected>Propietario</option>
                        <option selected>Arrendado</option>
                        <option selected>Otro</option>
                      </select>
                          
                          <div class="form-group">
                            <label for="Textarea1" class="text-light">Mensaje a enviar</label>
                            <textarea class="form-control" name="mensaje" id="Textarea1" rows="3"></textarea>
                          </div>
                          <br>  
                        <input type="button" class="btn btn-outline-light btn-block border-light text-light font-weight-bold" value="registrar" id="registrar">
                  </form>

addRegister.php

enter if (isset($_POST)) {
$nombre = $_POST['name_usu'];
$telefono = $_POST['phone_usu'];
$email = $_POST['nom_usu'];
$torreApto = $_POST['torreApto'];
$arrendado = $_POST['sel_apto'];
$mensaje = $_POST['mensaje'];
require("connect.php");

// script guardando en la base de datos 
$query = $con->prepare("INSERT INTO informacion(nombre,telefono,email,torreApto,arrendado,mensaje) VALUES (:nom, :tel, :ema, :torr, :arr, :men)");
$query->bindParam(":nom", $nombre);
$query->bindParam(":tel", $telefono);
$query->bindParam(":ema", $email);
$query->bindParam(":torr", $torreApto);
$query->bindParam(":arr", $arrendado);
$query->bindParam(":men", $mensaje);

//ejecuta el script 
$query->execute();
$con = null;
echo "ok";

}