If you want to add product(s) as an upsell to another product, you need to tag your main product with the correct tags in the following format:
upsell_product-handle|variant.id_discount-percentage
For example, if you want to add the Button Tufting Expandable Headboard as an upsell to a product with a 20% discount, the tag would be:
upsell_expandable-headboard|8761501941851_discount-20
When applying discounts, the value must be in 5% increments.
Product Handle
This can be found at the bottom of the admin page for your product, and is always in lowercase with any spaces replaced for hyphens. For example, your product "Expandable Headboard" has the product handle expandable-headboard
.
Click the "Edit website SEO" link at the foot of the product admin page:

Figure 1: The "Edit website SEO" button on the product admin page
Then copy the product handle from the expanded form:

Figure 2: The product handle
Variant ID
This can be found by clicking the "Edit" button in the variant panel of the product admin page:

Figure 3: The product variant panel
Then you need to copy the long number after the word "variants" in the page URL (don't include the forward slash):

Figure 4: The variant ID is in the page URL - in this case it is 8761501941851