<form autocomplete="off" @submit.prevent="anular">
    <div class="modal fade" tabindex="-1" role="dialog" aria-hidden="true" id="modal_anular">
        <div class="modal-dialog" role="document">
            <div class="modal-content">
                <div class="modal-header">
                    <h5 class="modal-title">Anular factura</h5>
                    <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                        <span aria-hidden="true">&times;</span>
                    </button>
                </div>
                <div class="modal-body">
                    <div class="form-group">
                        <div class="form-group form-group-default">
                            <label for="observacion_anular">Observaciones</label>
                            <textarea id="observacion_anular" class="form-control" rows="3" v-model="observacion_anular" required></textarea>
                        </div>
                    </div>
                </div>
                <div class="modal-footer">
                    <button type="button" class="btn btn-sm btn-secondary" data-dismiss="modal">Cerrar</button>
                    <button type="submit" class="btn btn-sm btn-primary">Enviar</button>
                </div>
            </div>
        </div>
    </div>
</form>