```
$ aws --profile foobar --endpoint-url http://localhost:9000 sts assume-role --policy '{"Version":"2012-10-17","Statement":[{"Sid":"Stmt1","Effect":"Allow","Action":"s3:*","Resource":"arn:aws:s3:::*"}]}' --role-arn arn:xxx:xxx:xxx:xxxx --role-session-name anything
{
    "AssumedRoleUser": {
        "Arn": ""
    },
    "Credentials": {
        "SecretAccessKey": "xbnWUoNKgFxi+uv3RI9UgqP3tULQMdI+Hj+4psd4",