Ideally, the slot should show up under the function app like below
Please update the function core tools and Azure function extension in the VS code if you don't see the slot.
If the slot doesn't show up even after upgrading the versions, please try to use the function core tools command to deploy to the function deployment slot. The name would be functionAppname-slotname (example: functiontest-dev)
func azure functionapp publish <FunctionAppslotName>