As of Motion v2.1 you can now specifically select related products on your individual product pages with the help of utility tags.
How does it work?
By default, related products only show up when you're on a collection-relative URL, such as /collections/your-collection/products/your-product. You can only get to this URL if you navigate to a product through a collection (or featured collection homepage section).
Motion's new utility tags allow you to show a related collection of your choosing, even when navigating directly to the product URL (/products/your-product).
Make sure "Show related products" is enabled in your Product pages settings (it is enabled by default).
1. Get your collection handle
First, you'll need the collection handle that you want to feature on your product. From your /admin/collections page, click "Edit website SEO" near the bottom of the page.
Select the text in the last field after /collections/.
2. Add the utility tag to your product
Navigate to the product you want to add the above collection to as related products. Add your utility tag in the format _related_collection-handle.
In this example it would be _related_menswear.