A company has an Office 365 tenant that has an Enterprise E1 subscription. You configure the policies required for self-service password reset. You need to ensure that all existing users can perform self-service password resets. Which Windows PowerShell cmdlet should you run?

Self-service password reset with on-premises write-back is a Premium-only feature.
Example:
The following command adds the Office 365 for enterprises license to the user.
Set-MsolUserLicense -UserPrincipalName [email protected] -AddLicenses “Contoso:ENTERPRISEPACK”
Note: The Set-MsolUserLicense cmdlet can be used to adjust the licenses for a user. This can include adding a new license, removing a license, updating the license options, or any combination of these actions.
References:
https://docs.microsoft.com/en-us/powershell/module/msonline/set-msoluserlicense?view=azureadps-1.0