20% of the minimum hourly rate per hour for each hour required to stand by
This is a type of on-call allowance. You will need to determine if this is part of the employee's ordinary hours or not to determine how to treat it.
Withhold tax from this allowance
If this is part of ordinary hours, it should be included in ordinary times earnings. If it is outside of ordinary hours, it should not be part of ordinary times earnings
Include this allowance for payroll tax purposes
Include this allowance as part of gross wages