At some point with a sick chicken, it will be necessary to tube feed due to the decline caused by dehydration and starvation. It can mean the difference between the patient surviving or dying.
You need to obtain the tubing kit. You can get one from a vet for a few dollars or order it here...
This is how to syringe fluids into a chicken without getting it into the airway at the back of the tongue. You will aim for the esophagus on the chicken's right side of their throat. It leads right into the crop.