I use Safeguard liquid (sold for goats), which is 10% Fenbendazole. You can apply it to little bits of bread and get the bird to eat it that way.
Usually best to dose by weight, if you can get a weight on your bird. (If it is tame enough to hold, you can step on and off a scale with and without the bird). You want 50 mg (of medicine) per kg (2.2 lb) of bird, which works out to 0.5 ml of the pre-mixed liquid for every kg (2.2 pounds) that your bird weighs. I measure it with a little dosing syringe like the ones used for infant or child liquid medicine. I draw up the amount and spread it on several bits of bread until its all on the bread.
Since your bird is underweight and not fully grown, it's hard to estimate how much it weighs. In general, the dosage for average peafowl varies from 1.5 ml for a small adult female, to 2 ml for a large adult female or small adult male, up to 3 ml for a fully grown male.
Another good wormer is albendazole. Let us know which ones you can find at your local stores and we can help with dosages. @casportpony is our resident pea health expert, and she will probably chime in shortly
Also, you may want to go ahead and treat for coccidia, since it's a new bird to you and is underweight. Check out what the bird poop looks like -- casportpony may want pictures, lol. For coccidia, you can use amprolium powder in the bird's water -- so pick some up at the store where you find the fenbendazole.
Make sure your bird has room to move around (100 sq ft minimum, more is better) and a perch for sleeping. Peas need much more room than chickens in order to be healthy.
Let us know how it goes and welcome to the peafowl forum !!!