At 2.1, call portal.GetUserContentAsync
var owner = portal.GetSignOnUsername();
var userContent = await portal.GetUserContentAsync(owner);
foreach (var pf in userContent.PortalFolders)
System.Diagnostics.Debug.WriteLine($"{pf.Name},{pf.FolderID}");
At 2.0, you have to execute the rest query directly for user content (via EsriHttpClient). The url would look like this:
var owner = portal.GetSignOnUsername();
var url = $"{portal.PortalUri.AbsoluteUri}/content/users/{owner}";
var response = new EsriHttpClient().Get(url);
var json = await response.Content.ReadAsStringAsync();
The returned json will contain your folders. See Rest API User Content