Populate value in a directory extension attribute on a cloud account

Question

We have configured directory extensions on our Azure AD Connect sync and created the extension I would like to populate a value in this attribute for a Azure AD cloud only account but getting an error. I've tried on both guest and member accounts. See details below.

Set-AzureADUserExtension -ObjectId " " -ExtensionName "extension attribute" -ExtensionValue "Admin@keyman .onmicrosoft.com"

Set-AzureADUserExtension: Error occurred while executing SetUser
Code: Request_BadRequest
Message: One or more property values specified are invalid.

Note : This works fine for user accounts synchronized from our on-premise Active Directory environment. But I have been unable to set this attribute for cloud only accounts. I do NOT want to create yet another new attribute.

My Question: Can any one please confirm if i can set the extension attribute for Cloud only account?