Barry Wood

I've heard I can claim a working from home allowance in my self-assessment tax return. I'm filling out my return for the year 20/21, but I don't see any mention of it anywhere. The only place I can see it mentioned is here: https://www.gov.uk/tax-relief-for-employees/working-at-home, where it instructs me that I need to include it in my tax return. But where?!