Can I provide SSH key after creation of Linode?

You can add an SSH key to your Linode a couple of ways, though the simplest would be using SCP from your computer after creating the correct directory on your Linode. You can also manually copy/paste the key into a file on your Linode if you are unable to use SCP.

Our Securing Your Server has a section on this, including the location of the key and how to further harden access to your Linode.