Add an Admin to Your Facebook Page

This is the process for adding an admin to your Facebook page (not through Facebook Business Manager)

1. Go to your Facebook page
2. Navigate to Settings in the left menu
   
3. Click Page Roles
   
   
4. Type the email address of the person you're adding and select the person from the list that appears
5. Choose admin from the dropdown
   
6. Click Add and enter your password to confirm.

Keep in mind that if you're not friends with the person you're adding, they'll have to accept your invite before they can start helping you manage your Page.

If you want to add another admin, the request may need to be approved by another page admin before it can be added. If the request is not actioned within 7 days it may be auto-approved.

Source: Manage roles for your Facebook Page