If you loaded the map through Content.Load already, you should have a reference to the TiledMap object. From there you can find all object layers through the public ReadOnlyCollection<TiledMapObjectLayer> ObjectLayers { get; }
property or use public T GetLayer<T>(string layerName) where T : TiledMapLayer;
with TiledMapObjectLayer to retrieve the layer you want.
As an example, I have a Tiled map with a dedicated collision layer. I have this property in a wrapper class:
public TiledMapObjectLayer CollisionLayer { get; private set; } = null;
To retrieve my collision layer, I simply do this: CollisionLayer = Map.GetLayer<TiledMapObjectLayer>(LevelGlobals.CollisionLayerName);
From there I have access to all the objects in the collision layer.