Assign Office365 Licenses using PowerShell and Graph API. Associated blog post can be found here https://blog.darrenjrobinson.com/adding-removing-user-office365-licences-using-powershell-and-the-azure-ad-graph-restapi/

AddLicenseToUsers.ps1

# License Body for RestAPI
 $body = @{
    addLicenses = @(@{"skuId" = $LicenceToAdd})
    removeLicenses= @()
}

# Convert it to JSON
$postbody = $body | ConvertTo-Json

# License Users
foreach ($usertolicense in $unlicencedUsers){
    $AssignLicense = Invoke-RestMethod -Method Post -Headers @{
        Authorization   = $authenticationResult.CreateAuthorizationHeader()
        'Content-Type'  = "application/json"
    } -body $postbody -Uri ("https://graph.windows.net/myorganization/users/$usertolicense`/assignLicense?api-version=1.6" -f $authenticationResult.TenantId)
}