I've just published custom filtering support for DropDown, DropDownDataGrid, AutoComplete and ListBox. Check the third DropDown in this demo:
https://blazor.radzen.com/dropdown
Here is the relevant code:
@inject NorthwindContext dbContext
...
<RadzenDropDown AllowClear="true" TValue="string"
LoadData="@LoadData" AllowFiltering="true"
Data="@customCustomersData"
TextProperty="CompanyName" ValueProperty="CustomerID" />
...
@code {
IEnumerable<Customer> customCustomersData;
void LoadData(LoadDataArgs args)
{
customCustomersData = dbContext.Customers.Where(c => c.CustomerID.Contains(args.Filter) || c.ContactName.Contains(args.Filter)).ToList();
}
}