Enter config terminal:

config t

Request certificate:

crypto pki enrol TRUSTPOINTADDRESSHERE

Now use the router hostname as the username and password to make things simple. In my case this was: bens-router

If you have more than one trustpoint you can repeat the process but change the trustpoint name in the enrol command above.

After you have finished check the status of the trustpoint with:

sh crypto pki trustpoint status