LA 200 is an injectable form of oxytetracycline, and is a long acting medicine for cattle given by injection every 3 days. I cannot tell you the correct dosage for a chicken. I would recommend getting tylosin for pigeons, and putting that in her water 1 tsp per gallon for 5 days. Or see a vet for an antibiotic. Doxycycline is another one that is frequently used.
Your chicken could just have a respiratory virus, which does not respond to antibiotics. If sneezing is the main symptom, that could be the problem. Aspergillosis from mold or wet conditions, could cause silent gasping, and that too does not respond to antibiotics. If you see bubbles or foam in an eye, swelling of the face, crackles or wheezes, then MG might be a problem. Here is a list of common diseases and symptoms:
https://edis.ifas.ufl.edu/ps044