Is there a sandbox to try External Data Source setting?

Hello,

Some of our clients are using Adobe. The clients will need to integrate with our system via REST API. I want to test how Adobe can define a connection with our system. This article seems the right place I should go: https://experienceleague.adobe.com/docs/journeys/using/data-source-journeys/external-data-sources.html 

I want to try the External Data Source setup to better understand the details. May I know if there is a sandbox is ready for this purpose?

BTW, I have just created my account on Adobe; however, I don't have permission to do anything after logging in. It shows me:

It seems that your Adobe ID is not linked with your Adobe Experience Cloud solution accounts.
Please get in touch with your Org Administrator or Adobe Enterprise Support.

Do we have to have Adobe Cloud solution account in order to try this feature? We are third-party service provider and we don't have Adobe Enterprise account. Any other option we can try the External Data Source connection?

Thanks,

Junsheng