To apply license to a standalone host you will have to go to Host>Configuration>Licensed features>edit and assign a key.
The ESXi works with 60-day evaluation. After which it will not work until you buy and apply a license. If you want to use a free ESXi you can download it here
Free VMware vSphere Hypervisor, Free Virtualization (ESXi) | United States
This can only be used as standalone and you cannot add it to vCenter. But the license is free.