Hi @Nico73 ,
Do you want to connect to an external SharePoint Site and in that tenant you have been invited as guest?
If you do, I'm afraid it's not supported to connect with external sharepoint list with internal account in powerapps. That should be sharepoint connector's limit.
You could only create connections with sharepoint with the internal account of the sharepoint site.
In short, you need to create connection by using CompanyB's account to connect with CompanyB's sharepoint list, then create this connection in CompanyA's PowerApps.
The suggestion is:
1)create an account in CompanyB
2)create connection with sharepoint by using CompanyB's account in CompanyA's powerapps
You could create external connections in connection management page:

Best regards,
Community Support Team _ Jeffer Ni
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