I believe you have a female (hen).

You will know soon enough if it starts laying. That’s great that you rescued her. To be honest, I’m not sure what to tell you. When my ducks were younger, I would sprinkle niacin (bought from Walmart in capsules) over their food. Also, I would put a little apple cider vinegar in their water. Mine are over a year old and free range on our pond. Now I feed them a mix of flock food and cracked corn to supplement their diet. Muscovies are amazing animals. We have three and love ours!