To claim your Apple Maps listing, go to https://mapsconnect.apple.com
You’re going to need an Apple ID, so if you don’t have one already, you’ll need to create one. You can then follow the steps on-screen to claim or add your business which typically includes a phone verification to ensure that you legitimately represent the business.
The Apple Maps interface isn’t as developed as Google and you can’t have multiple admins for each business. It’s important that you retain ownership of assets like this, which is why White Stone Marketing cannot complete this on your behalf. But you can have multiple locations associated with a single Apple ID.