I usually try random codes with the following format: COMPANYNAME15, COMPANYNAME20, etc. Every once in a while (maybe 10% of the time?) I get lucky.

WELCOME10, WELCOME20, etc also work a good amount of the time. A lot of the sign up for our mailing list and get a discount promo codes match up to WELCOME plus the percentage off.

"5000off" sounds like the discount code equivalent of "password" as a password. So probably just a wild guess as a joke but then it wasn't a joke.

> How did they find the code?

They had a MBP with a butterfly keyboard

Probably.

    for i := 0; i <= 999999; i++ {
        code = fmt.Sprintf("%doff", i)
    }

"X% off" codes are pretty common.

If 5OFF worked why not try 500/5000off?