The concept of a default bookmark is a pure client concept. The engine does not really know anything about this. There is a special app object created by the client that stores app metadata that the client requires such as some color schemes and theme information. This object also contains the ID of the default bookmark, and upon connection to the app, the client will read these properties and simply call ApplyBookmark with that ID.
There are several ways to access this client properties object, but I find the simplest one (though not the most efficient) is to do like this:
using (var app = location.App(appId))
{
var objectInfo = app.GetAllInfos().Single(info => info.Type == "appprops");
var clientAppPropsObject = app.GetGenericObject(objectInfo.Id);
var defaultBookmarkId = clientAppPropsObject.Properties.Get<string>("defaultBookmarkId");
Console.WriteLine(defaultBookmarkId);
}
The reason why I said this is not the most efficient, is that call to App.GetAllInfos really returns information about all app entities, and you're really looking for one single. Efficiency wise it's better to do the filtering server-side. You can do that either by creating a list session object, or use the filter capability of the repository API. This thread shows you how to get the app properties using a list object:
https://community.qlik.com/t5/New-to-Qlik-Sense/Qlik-sense-Net-SDK-app-theme-name/td-p/1738177
If you prefer to use the repository API, then you can do something like this with the following library https://www.nuget.org/packages/QlikSenseRestClient/
var client = new RestClient(url);
client.AsNtlmUserViaProxy();
var clientPropsObjectInfo = client.Get<JArray>($"/qrs/app/object?filter=app.id eq {appId} and objectType eq 'appprops'").First;
var clientPropsObjectId = clientPropsObjectInfo["id"].Value<string>();
Console.WriteLine(clientPropsObjectId);
