How to help an underweight chicken ?

[Peppercorngal]

UPDATE....she is full of lice or mites I don’t know what am I supposed to do

Oh No, sorry to hear that. Check out this website: https://the-chicken-chick.com/poultry-lice-and-mites-identification/.

 

[DobieLover]

UPDATE....she is full of lice or mites I don’t know what am I supposed to do

Have you treated her for the external parasites? You need to dust her with 10% permethrin.

 

[Magapoule]

Have you treated her for the external parasites? You need to dust her with 10% permethrin.

I treated her with permethrin but not 10%. It’s with Bronco, a horse fly spray that has permethrin in it and it worked !

 

[DobieLover]

I treated her with permethrin but not 10%. It’s with Bronco, a horse fly spray that has permethrin in it and it worked !

Excellent!!
I would now work on building her back up. She needs protein to help replenish her blood loss. I would get her Flock Raiser or any other complete feed with 18-20% protein. Make a mash out if it. They seem to like that better. For maybe 3 days, I'd put about 1/2 ml of Poultry Nutri-drench in the mash. Feeding that too long can cause diarrhea but the extra nutritional support for 3 days will help her out.
Hopefully she will regain her strength soon.
You should also repeat spraying her in 7 days then again in 14 days to make sure you kill anything that hatches from eggs on her. And clean and spray her cage.