We usually say you can choose any subdomain as your Marketo LP domain, as long as it’s not already used by your company: pages.example.com, lp.example.com, get.example.com, signup.example.com, whatever-sounds-right.example.com.
But in truth there are subdomains you definitely should not choose: promo.example.com, affiliate.example.com, and a few more.
Why? Because these subdomains are on the EasyList General block list — a database used by the popular AdBlock plugin and others of the same ilk. Here’s the relevant entry for promo.example.com on the list:
(It’s a strange-looking rule, because the maintainers of EasyList added exceptions for the Promo web app, who operate the domain promo.com — note that one’s not a subdomain of another domain — as well as Promo’s sister site Slidely and a couple of their customers.)
As a result, any request to promo.example.com from a document on another private domain will fail. The AdBlock debugger is at least clear about the reason:
Now, this will not affect a visit directly to https://promo.example.com (where that’s the domain in the browser’s Location bar). In that case, the first-party case, there’s no problem.
Nor will it be a problem if you fetch resources on https://promo.example.com from a site that shares the same private domain, like https://www.example.com.
The problem is that in order for your embedded forms to not be detected by Firefox’s Tracking Protection, as well as by 3rd-party plugins like AdBlock, you must load your forms from your Marketo LP domain. That’s because marketo.com is found elsewhere on EasyList — as a known tracker — and if you load from app-*.marketo.com instead, browsers will block the forms for other reasons.
So the perfect storm is when another of your sites, like https://www.example.io, loads embedded forms from https://promo.example.com. Or, of course, if you have a partner embedding your forms on https://channel.partner.example. Even if you’re the owner of both sites, from AdBlock’s standpoint, that’s a 3rd-party call, and the specific prohibition on the “promo” subdomain comes into play.
There are only a few other subdomains treated this strictly: “affiliate”, “affiliates”, and “affiliates2”, plus “banner” and “banners”. Stay far away from these!
Oh no! We’re already using promo.example.com!
The good news is if you’re already using promo.example.com or one of the other listed ones as your primary LP domain, you don’t have to switch that out. You can add a secondary Domain Alias instead, like forms.example.com, and use that one just for form embeds (not a bad idea to use a form-specific Alias anyway, as it makes the purpose clear).