Kunde:
<TelerikAutoComplete Data="@DataCustomer" TItem="@Customer" ValueChanged="@((string value) => OnComboCustomerValueChanged(value))">
    <ItemTemplate>
        @(nameof(Customer.customerName))
    </ItemTemplate>
    <FooterTemplate>
        <h6>Anzahl: @DataCustomer.Count()</h6>
    </FooterTemplate>
    <NoDataTemplate>
        <div class="no-data-template">
            <TelerikSvgIcon Icon="@SvgIcon.FilesError" Size="@ThemeConstants.SvgIcon.Size.Large"></TelerikSvgIcon>
            <p>No items available</p>
        </div>
    </NoDataTemplate>
</TelerikAutoComplete>

protected override async Task OnInitializedAsync()
{
    List<Customer> customers = new List<Customer>();
    customers.Add(new Customer()
        {
            customerId = "",
            customerName = ""
        });

    DataCustomer = customers;
}

 private void OnComboCustomerValueChanged(string value)
 {
     ComboCustomerValue = value;

     if (value != null && value.Length > 3)
     {
         if (DataCustomer == null)
         {
             List<Customer> Data = new List<Customer>();
         }

         if (DataCustomer.FirstOrDefault(item => item.customerId == value.ToString()) == null)
         {
             IO.Swagger.Api.CustomerApi customerApi = new IO.Swagger.Api.CustomerApi("https://server:8080");
             foreach (var itemsFound in customerApi.ApiV1CustomerIdGet(Convert.ToInt32(value)))
             {
                 if (DataCustomer.FirstOrDefault(item => item.customerId == itemsFound.Id.ToString()) == null)
                 {
                     DataCustomer.Add(new Customer()
                         {
                             customerId = itemsFound.Id.ToString(),
                             customerName = itemsFound.Name + @" (" + itemsFound.Id.ToString() + @")"
                         });
                 }
             }

         }
     }
 }

<p>@IsLoading</p>
<TelerikAutoComplete
    @ref="@AutoCompleteControl"
    Data="ActiveDataSet"
    @bind-Value="SearchTerm"
    DebounceDelay="500"
    Placeholder="Search here..."
    FilterOperator="StringFilterOperator.Contains"
    ClearButton="true">
    <NoDataTemplate>
        @if (IsLoading)
        {
            <TelerikLoader Visible="true"
                           Size="@ThemeConstants.Loader.Size.Large"
                           ThemeColor="@ThemeConstants.Loader.ThemeColor.Tertiary"
                           Type="@LoaderType.ConvergingSpinner"/>
        }
        else
        {
            <p>No Data</p>
        }
    </NoDataTemplate>
</TelerikAutoComplete>

private async Task Search(string searchTerm)
{
    try
    {
        // Long running search that updates the data
    }
    finally
    {
        IsLoading = false;
        StateHasChanged();
        // AutoCompleteControl.Rebind(); // Without this the loading spinner never goes away
    }
    
}