Ajax Call is sending empty request.POST or request.body

Hi i am still not clear when using request.POST or request.body.

Anyway this ajax call is sending emprty request.POST, or empty  request.body:

    <script type="text/javascript">
    $(document).ready(function(){

        $(".captcha_form").submit(function(event){
            var check = $('#id_check').val();
            alert(check);
            $.ajax({
                type: "POST", 
                url: "./captcha",
                contentType: "application/json",
                data:check,
                dataType: "json",
                success: function(data) {
                    alert("passa");
                    alert(data.status);
                   
                  },
                error: function(data) {
                    alert("NOT_OK");
                  }         
            });
       
    });
    });
   
    </script>


<form id ="captcha_form" class="captcha_form" action="./captcha" method="post">{% csrf_token %}
    {{form}}
<input type="submit" id="button" class="button" value="Invio"/>
</form>


class check_box_form(forms.Form):
    scelte = [('Yes','Yes'),('No','No')]
    check = forms.ChoiceField(label='I am a Robot       ',choices=scelte)

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