Update the companyName attribute for an Azure AD user via PowerShell

I had the problem and had to ask Microsoft directly. Since February 2019, CompanyName is writable.

Command is

Set-AzureADUserExtension -ObjectId <UPN> -ExtensionName "CompanyName" -ExtensionValue "<VALUE>"

Check my blogpost on the matter for the whole explanation: http://www.yasab.net/azure-ad-update-companyname/

How does one set the companyName attribute for users in Azure AD / Office 365?

For now, customer can use Azure AD connect to sync on-prem AD user's attribute company to Azure AD, but can't set company for cloud user, the attribute company is read only.

Here a similar case about you:

This attribute company is inherited from the Display name property of the organisation but is not visible in the Graph API directly.

As a workaround, you can create users to your on-prem AD, then sync to Azure AD.

About attribute extension_0000000000000000000000000000000_company, used for mapping attribute.