CustomerOrderDetailsViewModel.cs

internal class AwaitableMessages
{
    public static Task<T> NextMessageAsync<T>()
    {
        var tcs = new TaskCompletionSource<T>();
        Messenger.Default.Register<T>(null, item => tcs.TrySetResult(item));
        return tcs.Task;
    }
}

namespace MongoDBApp.ViewModels
{
    [ImplementPropertyChanged]
    public class CustomerOrdersViewModel : IPageViewModel
    {
        private IDataService<OrderModel> _orderDataService;
    
        public CustomerOrdersViewModel(IDataService<OrderModel> orderDataService)
        {
            this._orderDataService = orderDataService;
            this.Initialization = InitializeAsync();
        }

        #region properties

        public string SelectedCustomerEmail { get; set; }

        public ObservableCollection<OrderModel> CustomerOrders { get; set; }

        public Task Initialization { get; set; }

        #endregion

        #region methods
        
        private async Task InitializeAsync()
        {
            var customer = await AwaitableMessages.NextMessageAsync<CustomerModel>();
            SelectedCustomerEmail = customer.Email;
            await LoadCustomerOrdersAsync(SelectedCustomerEmail);
            IsEnabled = true;
        }

        public async Task LoadCustomerOrdersAsync(string email)
        {
            var ordersResult = await _orderDataService.GetAllByEmailAsync(email);
            CustomerOrders = ordersResult.ToObservableCollection();
        }

        #endregion
    }
}