This can only be done using eCart 5, it is not supported in eCart 4.
The link will need to be similar to a link to a detail page, where it passes the item ID as a query string variable for example:
on the add to cart page, create a recordset that filters the ID column on the passed query string variable.
once the recordset has been created, go to the Server Behaviors panel, Window -> Server Behaviors, and click the plus button. Select eCart -> Add to Cart -> Add to Cart. Set the trigger to the recordset not being empty. Set the Go To Page to the cart display page. Then go to the bindings page and bind the cart columns to the recordset columns.