Two Steps to Manage this Licensing for vCenter. Please note the same we can do for ESXi Hosts.

• Step#1 Add License Key to Virtual Center
• Step#2 Assign License to Virtual Center

Manage License Key for Virtual Center Server:-

Step#1 - Add License Key to Virtual Center

• Login to Virtual Center using Web or vSphere Client. Here we have logged in thrugh Web Client.
• On top of console, you can see the note saying that licenses in our inventory either expired or not available.
• Go to Home > Click on Licenses under Administration Tab

• Click on Manage your Licenses in Getting Started Tab or Go to Licenses Tab.
• Click on + icon to Create New Licenses.

• Enter the vCenter Server 6.x Standard license key and press Enter to add a new line.
• Enter the vSphere 6.x Standard license key(For ESXi), and click Next.

• Label the license keys as per your convinient. Click on Next.

• Your Licenses have been added here. Now let's move to Step#2 to Assign Licenses.

Step#2 - Assign License to Virtual Center

• Go to Licenses Option and and Click on Manage Your Assets under Getting Started tab. It will redirect you to Assets Tab.

• Under Assets Tab > Go to vCenter Server Systems (To Assign License to Virtual Center). Do the same for ESXi Host in Hosts Tab if you want to assign License to ESXi Host.
  
• Click on Firts Icon -> to Assign License

• Select License and Click on OK.

• vCenter Server is Licensed Now. Enjoy Managing the Inventory.