Submitting AJAX Post to Body

I’m trying to submit a form using ajax and send the data from a textarea to the body of the post request.

<form method="post">
   <textarea name="fkb_newdata" id="fkb_newdata" class="form-control mt-5" cols="30" rows="10">
      data...
      goes...
      here...
   </textarea>
   <button class="btn btn-primary mt-2" type="submit" id="fkb_savedata">
      Daten übernehmen
   </button>
</form>

using javascript I prevent the default behaviour of the browser and submit the data to a c# endpoint.

$("#fkb_savedata").on("click", function() {
   event.preventDefault();
   somefunctiontopreparethedata();

   $.ajax({
        url: "/backend/apiendpoint",
        type: "POST",
        data: {
            newdata: newdata // JSON.stringify(newdata)
        },
        dataType: "json",
        success: function(res) {
            // console.log(res);
            $("#fkb_savedata").text("Daten wurden gespeichert");
            $("#fkb_savedata").prop("disabled", true);
        },
        error: function(error) {
            console.log('error');
        }
   });
});

with both data and newdata I have the issue that when I inspect with the network tab that the data looks like an array

e.g. for newdata:

Array ( [newdata] => Array ( [0] => Array ( [0] => 001086 [1] => 30.42 ) [1] => Array ( [0] => 001188 [1] => 30.42 ) ) )

and for JSON.stringify(newdata)

Array ( [newdata] => [["001086","30.42"],["001188","30.42"]] )

Formdata in The Payload Tab:

newdata: [["001086","30.42"],["001188","30.42"]]

Is there a simple way to transform this array to propper json format for submitting to the api endpoint?

thank you in advance!