Find whatever you can manage with the highest calories. Protein/calorie shakes can be helpful. I also try to minimize activity and anything that is going to burn calories. Take calories anytime you can even if it's a small amount.
I also use cannabis to help encourage me to eat as I struggle with even wanting to eat. I have been around 100lbs before but I should be around 180lbs, currently 165lbs and trying to gain back 15 more.
Good luck