You can pay your Self Assessment bill through your PAYE tax code as long as all these apply:
1. You owe less than £3,000 on your tax bill.
2. you already pay tax through PAYE , eg you're an employee or you get a company pension.
3. You submitted your paper tax return by 31 October or your online tax return online by 30 December.
To select this option on SimpleTax,
Click on VALIDATE MY TAX RETURN
Click on YES to proceed
Select the YES radio button for the question below