Where your Amazon link goes on Facebook
On Facebook your Amazon link goes in the caption, which ends with #ad unless it already discloses. It can also go in the first comment.
Settings → Accounts & platforms → Facebook has a Links group (called Where to put your Amazon affiliate links in the older layout; click the Facebook card to open it). Your link goes in the caption, in the first comment, or both, plus Auto DM for people who comment a trigger word.
Amazon link in the caption
The Amazon link in the caption switch adds your raw Amazon affiliate link (amazon.com with your tracking ID) to the caption. If you're in Facebook's affiliate program, Facebook can show that link on the video itself, as a product link.
#ad on posts with an Amazon link
A Facebook post that carries an Amazon link ends its caption with #ad, right before the link, unless the caption already discloses (it says #ad, affiliate, commission, Amazon Associate, paid link or sponsored). It is on by default. To turn it off, use the Add #ad to Facebook posts with an Amazon link switch, in the same Links group as the link switch on the desktop (Settings → Accounts & platforms → Facebook) and under Where to put your Amazon affiliate links in the phone app's Facebook settings. It only applies while Amazon link in the caption is on; with that switch off, the #ad switch is locked. The post preview shows the #ad. YouTube and Pinterest are unchanged: they keep their own disclosure line (Where the Amazon disclosure appears).
Amazon link in the first comment
The Amazon link in the first comment switch comments your link right after the post goes live on Facebook (not on Instagram, where comment links aren't tappable). If it shows Reconnect Facebook to turn this on, your Facebook connection is older than Meta's approval: reconnect Facebook once. See First-comment link.
The caption link is always a plain amazon.com link with your tracking ID already on it, on purpose, so Facebook can still show its own product card.
The link goes on a post whose Amazon product you picked or confirmed, or that EverySqueeze is sure of. If it isn't sure, the app asks before posting, and automatic posts wait unless you chose otherwise — see Confirm a product to get your Amazon link. If you set your own link for a video, that link goes in the caption instead, exactly as you pasted it (an Amazon link gets your tracking ID if it has none) — see Use your own link.
One setting, every device
This choice is saved to your account, not to a browser. It applies to every Facebook post: Post now from the desktop Composer or the phone app, scheduled posts and automatic posts. Change it on your phone and the desktop uses it straight away, and the other way round. The Composer has no separate link switch; its Caption & link settings button opens this section.
A link you type into a caption yourself is taken out before posting, so a post never carries two links or an untagged one; the setting above adds the one tagged link.