Importing SSH Key : SSH Key is Invalid

When I try to import a key from the clipboard as described here:

I get

I see that…
“Imported keys must be in OpenSSH or PKCS#8 format”
…and…
“Additionally, at this time, imported SSH keys from Putty are not compatible.”
…but that still leaves some ambiguity.

Could you provide a sample file containing either of the supported formats?
In particular, I want to convert what is in my .ssh folder (which was generated by ssh-keygen).

I have discovered that what is wanted is the key file as produced by ssh-keygen.

In my case they are files found in ~/.ssh.
These files begin with:

----BEGIN OPENSSH PRIVATE KEY-----
...