I had exactly the same issue as well, the problem i found was that cloudability is using -u (which should be username and password), and instead of using name and password you put your api key in.
The trick is that you do not put in a password, so the ":" at the end is required for the curl command to hit the enpoint with the right details.
You basically do what it says in the document but with the : at the end in the quotes, its not the correct way to do it, but its how they enabled it on their backend side.
so in your example:
MyAPIKey = "12345678901234"
I would do
curl https://api.cloudability.com/v3/vendors -u '12345678901234:'
#Cloudability