Current we are able to add comments to transactions via Custom Functions but we are not able to add links which are useful to open transactions, etc. There is a functionality in the “Related Lists” that allows a URL / link to be added, it goes like this:

details = Map();
details.put("contact_name",{"value":customer.get("contact_name"),"isExternal":true,"link":"https://books.zoho.com/app#/contacts/" + customer.get("customer_id")});
details.put("company_name",{"value":customer.get("company_name"),"link":"https://books.zoho.com/app#/contacts/" + customer.get("contact_id")});
details.put("status",customer.get("status"));
details.put("source",customer.get("source"));

The same functionality should be available for comments so we can have links like the comments generated by Zoho shown below