Dynamic data source

Sorry Anonymous, but I not sure the Power BI will let you this because of the way the Credential are effectively stored.
Rather than storing a single credential for https://xxxxxxxxxxx.sharepoint.com/ the service will be trying to create one for each https://xxxxxxxxxxx.sharepoint.com/sites/xxxxx-"&Company.  The service may not allow the company to change because it leads to the credential change or the need for new credentials.  New Credentials will require you into the dataset settings to update.

The Web.Contents relative path feature is really for API calls.  It allows the main part of the web address to be stored for credential purposes while allowing the whole web address to change.  I think in your case https://xxxxxxxxxxx.sharepoint.com/sites/xxxxx-"&Company is part of the main address.

Do you really need it to be Dynamic or are you merely trying to create a PBI Template that asks for the new Site.