License an ESXi Host Without vCenter Server
If you are directly connected to the host through the vSphere Client, you can assign the host a license key.
1. From the vSphere Client, click the Configuration tab.
2. Under Software, click Licensed Features.
3. Click Edit.
4. Assign a license key.
a. Select Assign an existing license key to this hostand select a license key from the Product list as shown below
b. Select Assign a new license key to this host, click Enter Key, and enter a license key and an optional label for the license key. For screens refer to section here
5. Click OK.