I'm not an accountant, but for the US in 2021, that was Form 1040, Schedule 1: Additional Income and Adjustments to Income, Part 1: Additional Income, line 8: Other income, sub-line i: Activity not engaged in for profit income. You would just put $40 on that sub-line and add it to your adjusted gross income. If you made only $40 on it, I don't it can reasonably be called self-employment income, per Publication 535.
[0] https://www.irs.gov/instructions/i1040gi#en_US_2021_publink1...
[1] https://www.irs.gov/pub/irs-pdf/p535.pdf