You can use Azure PowerShell and specify tags at the point of creation.
$tags= @{ Owner = "me"; CostCentre= "Blah"; OtherTag= "Maui"}
New-AzWebAppSlot -ResourceGroupName Default-Web-WestUS -Name "ContosoSite" -AppServicePlan "ContosoServicePlan" -Slot "Slot001" -Tag $tags
ref: https://learn.microsoft.com/en-us/powershell/module/az.websites/new-azwebappslot?view=azps-10.4.1
It's kinda strange that the Azure CLI doesn't provide this, but it provides the ability to inherit configuration from another slot. https://learn.microsoft.com/en-us/cli/azure/webapp/deployment/slot?view=azure-cli-latest#az-webapp-deployment-slot-create