You can use the vSphere Web Client to access the ESXi license key.

Procedure
1

From the vSphere Web Client, connect to the vCenter Server.

2

Select the host in the inventory.

3

Select the Manage tab.

4

Select Settings.

5

Select System.

6

Select Licensing.

The license key appears in the form XXXXX-XXXXX-XXXXX-XXXXX-XXXXX.