All three work. It comes down to whether you want to send one link per post or several, and what converts best for your audience.
Caption only
Turn on Product Links on Facebook in Post Settings. Viewers tap the link in your post. The default caption doesn't include the "Comment {keyword}" prompt, so viewers won't know to comment for your links by DM. Here's which option to choose.
DMs only
Turn off Product Links on Facebook. Your caption includes the keyword prompt, and anyone who comments gets your links by DM, the same as on Instagram.
Does turning on Product Links on Facebook stop my DMs?
No, your DM automation keeps working. But when Product Links on Facebook is on, the default Facebook caption leaves out the "Comment {keyword}" prompt, so viewers won't know to comment your keyword, and they won't get your links by DM. To add the prompt back, turn on Use Custom Facebook Template and include {keyword}.
How do I use both?
Go to Post Settings and make sure Product Links on Facebook is on.
Turn on Use Custom Facebook Template.
Include both {product_link} and {keyword}. There's an example below.
Click Update Settings.
Get it on Amazon: {product_link}
To compare prices, comment "{keyword}" for the other product links!
{caption}
{hashtags}
{product_link} keeps your link, or your Shop button, in place, and {keyword} brings back the comment prompt. Any text you type around the variables always posts. Viewers who want the product now tap the link, and anyone who comments gets your Amazon link, TikTok link, Custom Link, or whichever are turned on for that video by DM.
If your caption link isn't Amazon, change the lead-in to match, for example "Get it here: {product_link}". With Native Link on the Video, the link is always Amazon, and removing {product_link} stops the Shop button from showing.
Which gets more sales?
A caption link sends one link with no extra step, which usually means a higher click-through rate.
DMs can send up to three links. Viewers have to comment first, so the ones who do are more interested, and the links stay in their inbox.
Results are mixed. Some creators see a much higher click-through rate from the caption link, and others get more sales from DMs. To test, pick one product and post a few similar videos with a caption link only and a few with DMs only. Compare them over the same number of days using Scaled Creator's click tracking and your Amazon Associates report. If you're not sure, start with the caption link, or run both.