just do all necessary steps ;-)
create a new Empty object
set instance_collection to the collection you want
set instance_type to 'COLLECTION'
link the resulting object to the place you want to have it..
(reference: Basic Object Operations Example)
source_collection = bpy.data.collections["the_name_of_the_collection_to_use"]
instance_obj = bpy.data.objects....
Thanks to batFINGER's comment, here is the working code: no need to pass arguments here, when calling operator : if it's stored in Scene, we can access to collection through context:
# Assign a collection
name = bpy.props.StringProperty(name="Test Prop", default="Unknown")