In APIs that authenticates with a single API key (eg a long random string) via Basic Auth, I have seen that most (eg Stripe, Unbounce) sends the API key as the username, leaving the password field blank. The only service I have seen that sends the API key in the password field is Bing.
Is there any reason to choose to send the API key as the username field or the password field?
I know that both usernames and passwords are concatenated and encoded, so both are equivalent in the transmission. I am looking for reasons such as (for example):
"Well known client X expects non-empty usernames, so the API key has to go in the username"
"Well known client Y logs usernames and not passwords, so the API key has to go in the password"