If you wanted nature to take its course you wouldn't be here looking for help 
Use a small 3ml syringe and you can drip it in the mouth a drop at a time or you can use this photo to syringe it in https://www.backyardchickens.com/threads/i-need-urgent-help-please.1198721/page-5#post-18974599

Use a small 3ml syringe and you can drip it in the mouth a drop at a time or you can use this photo to syringe it in https://www.backyardchickens.com/threads/i-need-urgent-help-please.1198721/page-5#post-18974599