public class InitTest : IInitTest
{
    public void Init()
    {
        var builder = WebApplication.CreateBuilder();

        builder.Services.AddControllers();

        var app = builder.Build();

        app.UseRouting();

        app.UseEndpoints(endpoints => 
        {
            endpoints.MapDefaultControllerRoute();
        });

        app.Run();
    }
}

public interface IInitTest
{
    void Init();
}

<ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
</ItemGroup>

using Microsoft.AspNetCore.Mvc;

namespace Embed.WebApi.Controllers
{
    [ApiController]
    [Route("api/[controller]/[action]")]
    public class TestController : ControllerBase
    {
        [HttpGet]
        public IActionResult Test()
        {
            return Ok("Hello World");
        }
    }
}

internal class Program
{
    static void Main(string[] args)
    {
        var services = new ServiceCollection();
        
        services.AddTransient<IInitTest, InitTest>();

        var serviceProvider = services.BuildServiceProvider();

        var initTest = serviceProvider.GetRequiredService<IInitTest>();

        initTest.Init();

        Console.Read();
    }
}

public class InitTest : IInitTest
{
    private static readonly string AssemblyName = typeof(InitTest).Assembly.GetName().Name;
    public void Init()
    {
        var builder = WebApplication.CreateBuilder();

        builder.Services.AddControllers()
            .AddApplicationPart(Assembly.Load(new AssemblyName(AssemblyName)));

        var app = builder.Build();

        app.UseRouting();

        app.UseEndpoints(endpoints => 
        {
            endpoints.MapDefaultControllerRoute();
        });

        app.Run();
    }
}

/// <summary>
/// Populates the given <paramref name="feature"/> using the list of
/// <see cref="IApplicationFeatureProvider{TFeature}"/>s configured on the
/// <see cref="ApplicationPartManager"/>.
/// </summary>
/// <typeparam name="TFeature">The type of the feature.</typeparam>
/// <param name="feature">The feature instance to populate.</param>
public void PopulateFeature<TFeature>(TFeature feature)
{
    if (feature == null)
    {
        throw new ArgumentNullException(nameof(feature));
    }

    foreach (var provider in FeatureProviders.OfType<IApplicationFeatureProvider<TFeature>>())
    {
        provider.PopulateFeature(ApplicationParts, feature);
    }
}

internal void PopulateDefaultParts(string entryAssemblyName)
{
    var assemblies = GetApplicationPartAssemblies(entryAssemblyName);

    var seenAssemblies = new HashSet<Assembly>();

    foreach (var assembly in assemblies)
    {
        if (!seenAssemblies.Add(assembly))
        {
            // "assemblies" may contain duplicate values, but we want unique ApplicationPart instances.
            // Note that we prefer using a HashSet over Distinct since the latter isn't
            // guaranteed to preserve the original ordering.
            continue;
        }

        var partFactory = ApplicationPartFactory.GetApplicationPartFactory(assembly);
        foreach (var applicationPart in partFactory.GetApplicationParts(assembly))
        {
            ApplicationParts.Add(applicationPart);
        }
    }
}

private static IEnumerable<Assembly> GetApplicationPartAssemblies(string entryAssemblyName)
{
    var entryAssembly = Assembly.Load(new AssemblyName(entryAssemblyName));

    // Use ApplicationPartAttribute to get the closure of direct or transitive dependencies
    // that reference MVC.
    var assembliesFromAttributes = entryAssembly.GetCustomAttributes<ApplicationPartAttribute>()
        .Select(name => Assembly.Load(name.AssemblyName))
        .OrderBy(assembly => assembly.FullName, StringComparer.Ordinal)
        .SelectMany(GetAssemblyClosure);

    // The SDK will not include the entry assembly as an application part. We'll explicitly list it
    // and have it appear before all other assemblies \ ApplicationParts.
    return GetAssemblyClosure(entryAssembly)
        .Concat(assembliesFromAttributes);
}

private static IEnumerable<Assembly> GetAssemblyClosure(Assembly assembly)
{
    yield return assembly;

    var relatedAssemblies = RelatedAssemblyAttribute.GetRelatedAssemblies(assembly, throwOnError: false)
        .OrderBy(assembly => assembly.FullName, StringComparer.Ordinal);

    foreach (var relatedAssembly in relatedAssemblies)
    {
        yield return relatedAssembly;
    }
}

[assembly: ApplicationPart("Embed.WebApi")]

<ItemGroup>
    <AssemblyAttribute Include="Microsoft.AspNetCore.Mvc.ApplicationParts.ApplicationPartAttribute">
        <_Parameter1>Embed.WebApi</_Parameter1>
    </AssemblyAttribute>
</ItemGroup>

<ItemGroup>
    <AssemblyAttribute Include="Microsoft.AspNetCore.Mvc.ApplicationParts.ApplicationPartAttribute">
        <_Parameter1>$(AssemblyName)</_Parameter1>
    </AssemblyAttribute>
</ItemGroup>