The Comment LINK line in Facebook captions
A line Facebook captions can open with, telling viewers what to comment to get your Auto DM.
When Auto DM is available, Settings → Accounts & platforms → Facebook, on its Auto DM tab, shows a switch labeled Add Comment LINK to captions when Auto DM is on, on by default.
What it does
Facebook captions start with a line like Comment LINK and I'll send you the link 👇 — using your first trigger word in upper case (or the post's own first trigger word, when it has its own Auto DM words) — whenever Auto DM is on, even before the post has a product: captions can't be changed after posting, so the line is ready for when you add one. Turn the switch off to stop adding the line, or change its wording below it. The line under the box, People will read, shows the result with your first trigger word in upper case. Your own wording must keep the trigger word exactly once, marked as {word}, hold no link, and stay under 120 characters — the link itself always goes in the DM, not in the caption.
When your Auto DM language is Español (where Spanish is on for your account; see Auto DM), the ready-made line is Spanish instead: Comenta ENLACE y te envío el enlace 👇. A line you wrote yourself stays as you wrote it.
Your caption stays whole under the line. Under the caption box in the Composer (and on a post's card in the mobile app) a note reads Facebook will open with: and the line, or says the line won't be added, with Change to open this setting. When the DM can't send yet, the note says why in one sentence, for example No product on this post yet: add one so the DM has a link to send.
When it's left out
The line is skipped when your caption already asks people to comment a trigger word, or when there's no room left in the caption.
It's added to the finished caption before your Amazon link line, after any caption cleanup.
On Instagram
Instagram's Auto DM settings (Settings → Accounts & platforms → Instagram, on its Auto DM tab) have their own choice, Add "Comment LINK" to Instagram captions, using the same wording. The line goes on an Instagram caption as soon as you switch on Auto DM on Instagram, even before it can send, so your posts are ready the day it starts. While it can't send, people who comment on Instagram don't get a DM, and the note under your caption says why. For example Reconnect Facebook so Auto DM can send. means your Facebook connection is older than Meta's approval of Instagram comments: reconnect Facebook once. It never holds a link: Instagram captions can't.