Insert specific collection into custom liquid section?

[UK_Aspire_Vendo]

Hey folks! I hope everyone is well.

We have a section of custom liquid on our site that suggests an upsell (the offer is that a free item can be obtained if a qualifying item is purchased)

At the moment in my liquid it informs the customers that it is available, but with just a text link to said free items (there's a few to choose from).

I don't think this is good enough, and I'd like to drop a collection into the custom liquid so the customer can see the range of free items on show, but I can't fathom how to produce the required collection. I can get shopify to produce A section, but it's empty and not what I'm looking for.

I'm not 100% what other information I can supply, so if someone could assist me I'd appreciate it!

For clarity, here is a link to a page where the free upsell is shown, it's the "free 10ml" dropdown.

I'd like to insert the collection into that dropdown so when the customer taps on it, all the relevent products are shown.

Thanks so much!
Dave