public string[] LayerMasks {get; set;}

{
  // Note: Needs QueuedTask to run
  {
    //Get the layer to be masked
    var lineLyrToBeMasked = MapView.Active.Map.Layers.FirstOrDefault(lyr => lyr.Name == "TestLine") as FeatureLayer;
    //Get the layer's definition
    var lyrDefn = lineLyrToBeMasked.GetDefinition();
    //Create an array of Masking layers (polygon only)
    //Set the LayerMasks property of the Masked layer
    lyrDefn.LayerMasks = ["CIMPATH=map3/testpoly.xml"];
    //Re-set the Masked layer's definition
    lineLyrToBeMasked.SetDefinition(lyrDefn);
  }
}