If you go to the My Settings tab, you should choose to have "Authentication" active -- and you will find that the sender is actually "ccsend.com"
So a simple TXT record like:
v=spf1 mx ptr include:ccsend.com ~all
Should probably handle it.
Of course, you can use the SPF Wizard to get a TXT record appropriate for your domain.
This thread is the first result in Bing and Google searches when looking for "constant contact spf" and because it's so outdated it really needs taken down because it's now WRONG information. The first response in the thread recommends including ccsend.com but that isn't even mentioned in the knowledgebase article
The knowledgebase states to include constantcontact.com but nothing is said about ccsend. According to openspf.org, this would likely cause an SPF check against your domain to fail, but even if it didn't, it's highly recommended to not use too many 'include' mechanisms: "SPF implementations MUST limit the number of mechanisms and modifiers that do DNS lookups to at most 10 per SPF check, including any lookups caused by the use of the "include" mechanism or the "redirect" modifier"
I think what I am seeing is that both may work.
I did finally find some information here that describes the authentication you describe.
Constant Contact Authentication
From email address and Sender Header address.
It looks like once CC Authentication turned on, emails are constructed so that the email will still appear to to the recipient to come from your domain, and a reply will be sent directly to you, but in the email header (that the reader doesn't see) that SPF type authentication uses, they will embed a different sending email address domain, one that will be confirmed by CC as a valid match. This would not require any specific SPF inclusion on our part at all, but be handled entirely by CC.
I expect that both solutions may work just fine.
Thank you for reaching out about this post coming up as the first result on your search. We don't have control over what posts will pull from searches in external tools, so we always recommend using the Knowledgebase directly, to make sure you're seeing the most up to date information. I apologize for any confusion this may have caused. It sounds like you were able to find the information you needed but we do have separate articles regarding self publishing and the Authentication option if you'd like to learn more. If you have additional questions, please reach out to our Account Review team directly, and they'll be more than happy to address any concerns!
Thanks for the reply, Amber. It's just that the interwebs are becoming very cluttered with bad (read: outdated) information so, in a way, you can control what appears in a search engine by removing this thread. Again, it's wrong information so why have it out there? The thread is 11 years old. I don't recall the last time I've read tech information from 11 years ago that's still relevant today.