class MessageSerializer(serializers.ModelSerializer):
normaluser = MessageUserSerializer(many=False, read_only=True) <- Esta linha
class Meta:
    model = Message
    fields = ('id', 'body', 'normaluser',)

<form method="post" id="msg">
    <div class="form-group">
        {% csrf_token %}
        <label for="comment">Message:</label>
        <input name="body"></input>
        <input name="normaluser" hidden value= {{ normaluser.id }}></input>
    </div>
     {{ form.as_p }}
    <button type="submit" onclick="sendMessage()">Enter</button>
</form>

function sendMessage() {
    var form = document.getElementById("msg");
    var csrf_token = jQuery("input[name=csrfmiddlewaretoken]").val();
    var req = new XMLHttpRequest();
    var url = '/website/message/';
    var method = "POST";
    var data = {body: form.body.value,
                normaluser: form.normaluser.value,};
    req.open(method, url);
    req.setRequestHeader("X-CSRFToken", csrf_token);
    req.setRequestHeader("Accept", "application/json");
    req.setRequestHeader("Content-Type", "application/json");
    req.addEventListener("load", function () {
        console.log(this.responseText);
        getMessages();
    });
    req.send(JSON.stringify(data));
    console.log(data);