What a form key is and where it goes
The public key that identifies one form, and the address you post to.
Every form has a public key. It looks like f_ followed by 8 letters or digits. The key is not a password: it only says which form a lead belongs to.
Where it goes
Send a POST to /f/<form key> on your ContactForms address. An HTML form uses the address as its action. A script uses fetch with a JSON body.
What happens to the lead
The lead is stored first. The route answers success only after the store is confirmed. If it is not known whether the lead was stored, you get a 503 unavailable answer and never a false success.
If the key is wrong
A path that is not a form key answers 404 form_not_found. Check that the key was copied whole, with no spaces.
More detail is in Send leads to a form URL.