Tested with the following demo client console app using .NET 7
using Synology.Api.Client;
var api = new SynologyClient("http://dsm-url.local");
await api.LoginAsync("username", @"password");
await api.FileStationApi()
.CreateFolderEndpoint()
.CreateAsync(new[] { "/folder/test" }, true);
Tried to follow same conventions.
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Tested with the following demo client console app using .NET 7