You could try giving a dose of that. And clean the umbilical area with something anti septic.
It won't hurt to try. Personally I'd mix the antibiotic a little stronger than is recommended I've done it before for bacterial infection but never with a chick... Best of luck
https://www.backyardchickens.com/a/mushy-chick-disease-yolk-sack-infection-omphalitis
I think it is a yoke sack infection also called mushy chick disease check out the link for info/help