Base URL used for Multi Tenants

OreSpurt 

It sounds like you are trying to use the Microsoft OneDrive for Business API to allow users to select files from their personal and company drives.

To authenticate users from multiple tenants, you will need to use the OAuth 2.0 protocol and obtain authorization credentials for each user.

The base URL for the OAuth 2.0 server is https://login.microsoftonline.com/common/oauth2/v2.0/authorize.

You can use this URL to redirect users to the Microsoft authorization server where they can grant your application access to their OneDrive data.

Once the user has granted access, you will receive an authorization code that you can exchange for an access token and a refresh token.

You can then use the access token to make authorized API requests on behalf of the user.

For more information on how to implement OAuth 2.0 for multiple tenants with OneDrive for Business, please refer to the Microsoft Graph documentation.

I hope this helps!