For the Test Target Name, you can type either a search string for the name of an existing host target or leave the field empty, click Search (magnifying glass icon), then select an appropriate host target.
Sadly doesn't work for me. I'm guessing Cloud Control cannot find the host target until the SSH credential has been created.
Or the server has not been added to the DNS. Can you ping the server (by name) from the OMS server? If not ask a network Administrator.
Currently in EM12c, an SSH key credential test can only be performed against an existing (discovered) target.
When I add a target manually... I just setup the keyswapping at the cli first then just ssh to test from there.
That way I know that swapping is working at the os level at the very least.