Set a Schedule

At the beginning of each tax year you will need to set the PaySchedule .
Start by getting a list of all PaySchedules.
The returned list will include an entry for every possible PayPeriod (Monthly, Weekly, etc) regardless of whether they're in use or not.
You're only interested in those where isRequired is true. The isRequired property indicates that you have Employees to pay in this tax year using this schedule.
You won't be able to start any PayRuns until you've set the schedule and the read-only property isConfigured returns true.

Set the Dates

Only two properties of a PaySchedule can be written to. They are firstPeriodEndDate and firstPaymentDate.
The value of firstPaymentDate determines which Tax Week or Month the first payment relates to.
firstPeriodEndDate specifies which period the payment is for.

So if you're setting your Monthly schedule for the 2018/19 tax year and your first payment will be on 1st May for the previous calendar month you'd update the PaySchedule at /employers/{employerId}/schedules/2018/Monthly with the following:

{
  "firstPeriodEndDate": "2018-04-30",
  "firstPaymentDate": "2018-05-01"
}

Repeat this process for each PaySchedule where isRequired is true and isConfigured is false.