Get a keyword list.
Use footprints: inurl:contact inurl:message inurl:talk - or whatever words you think will be present in the contact us URL. Merge your KWs and footprints.
Run your scrape.
Remove duplicate urls. Perhaps trim the urls to first folder or trim in such a way to get rid of any garbage at the end of the url.
Remove urls containing: wiki, wordpress, blogspot, yelp, yellow, and any other of these big sites. If you don't remove them it will be a pain later on.
Use the scrapebox page scanner to scan the pages for notorious contact elements like send button, submit button etc.
Once you have scanned the pages, remove any duplicate urls again.
Then send your contact form poster out to send the messages.
The heavy lifting here is done by the page scanner. You're using the harvester to grad a bunch of urls, but you will need to filter them with the page scanner to get the best results. This is a multi-step process and it will take some time to get right. I know because I spent a lot of time several years ago doing this exact thing. I haven't tried to do it for a long time, but I listed my basic process above.
Good luck!