You may create a user using the AWS CLI command
aws iam create-user --user-name Bob
When you do this, do not forget that you are referencing the iam endpoint rather than the s3 endpoint here. The command would look something like this:
$ aws iam create-user --user-name mikeotestuser--endpoint-url=https://iam.wasabisys.com
{
"User": {
"UserName": "mikeotestuser",
"Path": "/",
"CreateDate": "2019-09-18T16:04:37.031Z",
"UserId": "1PIFRFBITH1515N0TR3AL",
"Arn": "arn:aws:iam::100009912345:user/mikeo-test-iam"
}
}
$