It is better to share only required details. In your case it is fine to share Subscription Id to the company as they are in touch with you.
Azure subscription Id is usually not treated as PII or secret because with just the Id, nobody can log into it or know what is subscription name or who are the users / owners etc.
See similar question answered - https://stackoverflow.com/questions/45661109/are-azure-subscription-id-aad-tenant-id-and-aad-app-client-id-considered-secre
Please don't forget to Accept Answer and Up-vote if the response helped -- Vaibhav