[Parameter]
public RenderFragment<ListBoxEmptyDataAreaTemplateContext> EmptyDataAreaTemplate { get; set; }

@inject NwindDataService NwindDataService
<DxListBox TData="@WebApiLookup" TValue="string"
           ListRenderMode="ListRenderMode.Virtual"
           CssClass="cw-400 chi-220"
           CustomData="@LoadCustomData"
           @key="ComponentKey">
    <EmptyDataAreaTemplate>
        @if (context.IsDataLoading) {
            <div class="empty-data-area-template">
                <div class="d-flex flex-column">
                    <DxWaitIndicator Visible="true"
                                     CssClass="m-auto"
                                     AnimationType="WaitIndicatorAnimationType.Spin" />
                    <p class="dxbl-text d-block mt-1">Loading, please wait...</p>
                </div>
            </div>
        }
    </EmptyDataAreaTemplate>
</DxListBox>

@code {
    [Inject] protected HttpClient Client { get; set; }
    Guid ComponentKey { get; set; } = Guid.NewGuid();
    async Task<LoadResult> LoadCustomData(DataSourceLoadOptionsBase options, CancellationToken cancellationToken) {
        using var response = await Client.GetAsync(options.ConvertToGetRequestUri
            ("https://js.devexpress.com/Demos/NetCore/api/DataGridWebApi/CustomersLookup"), cancellationToken);
        response.EnsureSuccessStatusCode();
        if(options.RequireTotalCount)
            await Task.Delay(3000, cancellationToken);
        using var responseStream = await response.Content.ReadAsStreamAsync();
        var result = await JsonSerializer.DeserializeAsync<LoadResult>(responseStream, cancellationToken: cancellationToken);
        return result;
    }
    void ReloadListBox() {
        ComponentKey = Guid.NewGuid();
    }
    public class WebApiLookup {
        public string Text { get; set; }
        public string Value { get; set; }
    }
}

.empty-data-area-template {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 100%;
    height: 100%
}

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

public partial class Employee {
    public Employee() {
        this.Orders = new List<Order>();
    }
    public int EmployeeId { get; set; }
    public string LastName { get; set; }
    public string FirstName { get; set; }
    public string Title { get; set; }
    public string TitleOfCourtesy { get; set; }
    public Nullable BirthDate { get; set; }
    public Nullable HireDate { get; set; }
    public string Address { get; set; }
    public string City { get; set; }
    public string Region { get; set; }
    public string PostalCode { get; set; }
    public string Country { get; set; }
    public string HomePhone { get; set; }
    public string Extension { get; set; }
    public byte[] Photo { get; set; }
    public string Notes { get; set; }
    public Nullable<int> ReportsTo { get; set; }
    public string PhotoPath { get; set; }
    public string GroupName { get; set; }
    public virtual ICollection<Order> Orders { get; set; }
    public string Text => $"{FirstName} {LastName} ({Title})";
    public string FullName => $"{FirstName} {LastName}";
    public string ImageFileName => $"employees/{EmployeeId}.jpg";
}

using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using BlazorDemo.Data.Northwind;
using BlazorDemo.DataProviders;

namespace BlazorDemo.Services {
    public partial class NwindDataService {
        public Task<IEnumerable<Employee>> GetEmployeesAsync(CancellationToken ct = default) {
            // Return your data here
        }
    }
}

public class Startup {
    public void ConfigureServices(IServiceCollection services) {
        // ...
        services.AddScoped<NwindDataService>();
    }
}