Business Manager ID
Your Business Manager ID is a 15-16 digit number that uniquely identifies your Meta Business Manager. You share it with partners (like FexAds) to grant them partner access to your account. Find it in Business Manager > Settings > Business Settings, just below your business name.
The ID is required to set up partner relationships. The partner cannot find your Business Manager from your personal Facebook account or your business email — only from the Business Manager ID.
It is safe to share publicly. The ID alone does not grant any access. The owner of the Business Manager has to explicitly grant partner permissions inside their own settings.