When the email address is not available yet for the prospective candidate you see a greyed out button when trying to send a message.

In order to send the one time outreach message through the pop up profile, you need to click 'Get contacts' first to let HeroHunt.ai find a verified email address.

After HeroHunt.ai has found the verified email address, you can send the autogenerated message to the candidate.

You can of course also use Engage to find email addresses and send messages completely automatically instead of having to send the messages one by one.