How to post nested object to web service React-Native

My code it's work in operation 'delete'

  var params = {
             operation: 'delete',
             sessionName: state.params.sessionName,
             id: state.params.id
      };

      var formData = new FormData();
      const { navigate } = this.props.navigation;

      for (var k in params) {
        formData.append(k, params[k]);
      }

      fetch(state.params.u_url+'/webservice.php',
        {
          method: 'POST',
          body: formData })
       .then((response) => response.json())
          .then((responseJson) => {
            if(responseJson.success == false){
                alert (responseJson.error.message)
              return false;
            }

            // alert (responseJson.result.sessionName)
             this.props.navigation.goBack();


          })
          .catch((error) => {
              console.error(error);
          });

it's don't have nested object


but operation 'create' it's must to have nested object how can i create nested object

operation 'create' in postman I can use key = element

value = {"firstname":"Shawn","lastname":"Michel","email":"sm@wwe.com","mobile":"080012347","assigned_user_id": "19x1" }

My postman value it's work


  onAddBTN = () => {
      const {state} = this.props.navigation;

      var params = {
             operation: 'create',
             sessionName: state.params.sessionName,
             element: {
               firstname: this.state.firstname,
               lastname: this.state.lastname,
               email: this.state.email,
               mobile: this.state.phone,
               assigned_user_id:"19x1"
             },
             elementType: state.params.module,
      };

      var formData = new FormData();
      const { navigate } = this.props.navigation;

      for (var k in params) {
        formData.append(k, params[k]);
      }

      fetch(state.params.u_url+'/webservice.php',
        {
          method: 'POST',
          body: formData })
       .then((response) => response.json())
          .then((responseJson) => {
            if(responseJson.success == false){
                alert (responseJson.error.message)
              return false;
            }

            // alert (responseJson.result.sessionName)
             this.props.navigation.goBack();


          })
          .catch((error) => {
              console.log(element);
              console.error(error);
          });

  }

And I try to insert [] for array but it's not work :(

element: [{
               firstname: this.state.firstname,
               lastname: this.state.lastname,
               email: this.state.email,
               mobile: this.state.phone,
               assigned_user_id:"19x1"
             }],