I have downloaded the evaluation version for esxi 6.5 how do I get a license assigned to it after 60 days. I believe that the hypervisor is free version. Please guide.
Logon to the site on which you've registered for the free Hypervisor (likely https://www.vmware.com/go/get-free-esxi). Once logged in you can find your license key there.
Please see below link which explains how to get a free ESXi license key.
It is also worth reading the limitations at the top of the page, to ensure the free key will be suitable for you.