The best bet is to hide the pill in your chicken's most FAVORITE treat. A piece of cheese, or cooked egg, or a piece of meat? Something you can stick the pill into.
If she still won't take it, you may have to force it. Only do this if you have confidence. If the pill is small enough to easily swallow, it can be left whole. Otherwise you may need to break it up.
This is easiest with someone helping you. Wrap the bird in a towel, in such a way that the wings are held gently but firmly against the bird's sides. Think a chicken 'burrito' with her head sticking out one end. You want it to be firm but not tight enough to restrict her breathing!
Have your helper hold her with her facing you. Open her mouth; you can gently pull the wattles down, or touch the corners of her beak. Whatever works that won't hurt her. Make sure the fingernail is cut short and not ragged on your smallest finger (or use a latex glove, but it will taste terrible for the chicken!), and use it to gently push the pill as far as it will to to YOUR left (the hen's right) inside of her mouth. That is where the esophagus is. Her trachea is right in the MIDDLE, behind her tongue.
The esophagus on a chicken is very flexible and it's okay to use your finger to gently push the pill down it a little ways. This will make sure she does not choke on it. Remember, the esophagus is all the way to YOUR LEFT when you are looking at a chicken face on.
If you have to do this on your own, gently straddle the towel-wrapped chicken. Gently!! use your legs to hold her in place. You will both be facing the same way, now. So now, the esophagus will be on your right, and also the bird's right.
When the pill is inserted, unwrap your bird and allow them to recover, as this is stressful.
This method should not be used on chickens that are too small to pill this way (chicks, very small bantams, etc).