Military disability retirement pay you receive as a pension, annuity or similar allowance for a personal injury or sickness resulting from active service should not be counted in your taxable income. The same treatment covers VA disability compensation and pension payments to veterans or their families, grants for a home designed for wheelchair living, grants for a motor vehicle if you lost your sight or the use of a limb, and benefits under a dependent-care assistance program.
There is nothing to apply for. Leave these payments off the income you report on your Pennsylvania PA-40 return.
Your VA award letter showing the payments and what they are for.