Custom properties from Active Directory or Azure AD are not synchronized with SharePoint Online. They need to be imported or updated manually (using PowerShell for example).
You can sync your data from On-Premises Active Directory to Azure Active Directory (AAD) using Azure AD Connect, and then sync data from AAD to User Profile Service Application.
After that, you will be able to map AD field "extensionAttribute7" to custom field "SAPID" in AAD.
Detailed information and tutorial can be found via this article: How to Sync Azure AD extension attribute with User Profile for custom property in Office 365?
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