public string GetObjectIDField()

public async Task ObtainingDefinitionsFromFS(string url)
{
    //Url examples for (federated) feature services
    //(by "ref" online):
    //https://sampleserver6.arcgisonline.com/arcgis/rest/services/LocalGovernment/Recreation/FeatureServer
    //(federated by ref on portal)
    //https://portal.example.com/server/rest/services/FeatureServiceName/FeatureServer
    //(federated by value - Hosted - on portal)
    //https://portal.example.com/server/rest/services/Hosted/FeatureServiceName/FeatureServer

    await QueuedTask.Run(() =>
    {
        var uri = new Uri(url, UriKind.Absolute);
        using (var fs_db = new ArcGIS.Core.Data.Geodatabase(new ServiceConnectionProperties(uri)))
        {
            //Assuming that id 0 is a FeatureClass for the given service
            FeatureClassDefinition featureClassDefinition =
                    fs_db.GetDefinition<FeatureClassDefinition>("0");
            string shapeField = featureClassDefinition.GetShapeField();
            IReadOnlyList<Field> fields = featureClassDefinition.GetFields();

            //Assuming that id 1 is a Table for the given service
            TableDefinition tableDefinition =
                        fs_db.GetDefinition<TableDefinition>("1");
            string objectIDField = tableDefinition.GetObjectIDField();
        }
    });
}