If I recall from my SF days (its been a few years) it can not differentiate between a record and a folder that way.  You might have to ping SF support about this.  However, I have had a similar problem with Slack (a SF product btw) to Box but solved it, not using Relay or the Slack API, but by using Zapier.  Zapier  Box and can exploit any element of the API that other platforms cannot.  In my case, I wanted it so that when a user created a new channel in Slack, a new Box folder would created but ONLY if the channel named started with #project and not just any channel.  Zapier does this for me perfectly and its a transparent handoff.  I took a quick look to see if Zapier could do this for you and it can so might be worth a try if SF cannot help on the native integration.

 @n.shigemi since you are new to Box and Salesforce, you may also be interested in our bi-monthly community series with our product expects on Box for Salesforce. Sign up here

Hi @n.shigemi!

It sounds like an autolaunched flow tied to a custom button could do the trick for you here. 

You can use the custom button to pass the ID of the record to the flow, store it in a flow variable, and then use Box Flow actions in the same way as your original flow.

In the examples shown in my screenshots below, I am using a custom button on the Account object, and passing the Account ID into the flow, but it will work with just about any object you need.

More information on the Salesforce-specific aspects of that approach here: https://trailhead.salesforce.com/content/learn/modules/autolaunched-scheduled-flows/run-autolaunched-flow-from-custom-button