You can still reach out to an opportunity even if it doesn’t have a contact! While personalizing, open the site in a new tab and find the contact form URL. Add the contact form URL while personalizing by checking the 'Email not found, but this site has a contact form' box and a new field will pop up. Copy and paste the contact form URL into the field.
When composing, copy your template into your email like normal and click 'Next'.
You will be taken to a screen that has your email template and a button that says 'Copy Body to Clipboard and Open Form'. Click this button to open the contact form in a new tab and copy in your message.
When you are done, submit the contact form and click the 'Mark as Sent' button. This will add your message as a sent opportunity in Pitchbox.