Create a Content App in Umbraco using C# rather than package.manifest. For documentation see the website (https://bit.ly/3jbH1zE), the PDF (https://bit.ly/34t2Iqz) and the YouTube video (https://bit.ly/2EsUELE). You have the code-first or configuration-first approaches. The configuration-first approach uses the package.manifest file. Code-first …

MyContentAppFactory_v7.cs

using System.Web.Http.Filters;
using Umbraco.Core.Models.ContentEditing;
using Umbraco.Web.Editors;
using Umbraco.Web.Models.ContentEditing;

public class MyContentAppFactory
{
	// Works in v7
	public void Initialize() =>
		// on startup bind to this event
		EditorModelEventManager.SendingContentModel += EditorModelEventManager_SendingContentModel1;

	// Event handler
	private void EditorModelEventManager_SendingContentModel1(
		HttpActionExecutedContext sender,
		EditorModelEventArgs<ContentItemDisplay> e)
	{
		var apps = e.Model.ContentApps.ToList(); // get current apps list

		// TODO: I could add/remove/modify content app based on server side logic

		// Example - add a custom content app before the "Info" app
		apps.Insert(apps.Count - 1, new ContentApp
		{
			Alias = "dynamicApp",
			Name = "Dynamic App",
			Icon = "icon-flash",
			View = "../App_Plugins/MyContentApp/dynamic.html",
			// TODO: I could change the model based on server side logic
			ViewModel = new
			{
				name = "Hello",
				description = "I am a dynamic content app"
			}
		});
		//Re-assign
		e.Model.ContentApps = apps;
	}
}

MyContentAppFactory_v8.cs

using System.Collections.Generic;
using Umbraco.Core.Models;
using Umbraco.Core.Models.ContentEditing;
using Umbraco.Core.Models.Membership;

public class MyContentAppFactory : IContentAppFactory
{
	public ContentApp GetContentAppFor(object source, IEnumerable<IReadOnlyUserGroup> userGroups)
	{
		// I have access to the current IContent
		if (!(source is IContent content))
			return null;

		// TODO: I could add/remove/modify apps based on server side logic

		return new ContentApp
		{
			Alias = "myForm",
			Name = "My Form",
			Icon = "icon-edit",
			View = "../App_Plugins/MyContentApp/myForm.html",
			// TODO: I could change the model based on server side logic
			ViewModel = new
			{
				name = "Hello",
				description = "I am a dynamic content app"
			}
		};
	}
}

package.manifest.json

{
  "dashboards": [
    {
      "name": "My Form",
      "alias": "myForm",
      "view": "~/App_Plugins/MyContentAPp/myForm.html",
      "icon": "icon-edit",
      "show": [ "+content/*" ]
    }
}

Added Umbraco v7 and Umbraco v8 implementations of Content App using code-first approach with C#, and the configuration-first approach using the package.manifest