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 SAVE & CONTINUE
Click on YES to proceed