Call the method on the appfolder
object, instead of the KnownFolder
you're using.
Like this:
StorageFolder appfolder = await Windows.Storage.KnownFolders.DocumentsLibrary.CreateFolderAsync("Appdata.abc", Windows.Storage.CreationCollisionOption.ReplaceExisting);
StorageFile file = await appfolder.CreateFileAsync(DateFileName + ".uvraj", Windows.Storage.CreationCollisionOption.ReplaceExisting);