How do I mount iMac without VESA stand?

You can not remove the stand to install a VESA mount, but you can use a VESA mount that bolts to the stand.

for example: https://www.amazon.com/VIVO-Adapter-Bracket-Computer-Stand-MACB/dp/B00RW9F336?gQT=2

Got it, iMacs have not had removable stands since 2011.

You look on Amazon for an accessory that will work for your needs. You did not indicate which iMac. you own so, when looking on Amazon ensure the VESA mounts you are considering work with your model year and display size of the iMac you own.

By the way, if the iMac you have is new (meaning it is less than 2 weeks old and was ordered direct from Apple) just return it and buy the iMac with a VESA stand directly from Apple. They sell them on their on-line store.