I want to link expense(custom object ) entries with Activity object[Possible duplicate but need workarounds]

Question

Business requirement is to link existing custom object(Expense) to Activity(Task/Event). So that they can leverage capabilities of Activity object and link all expenses to an activity.

As per salesforce documentation and following links direct creation of lookup to activity(task/event) is not possible. How can I solve this business problem in most efficient manner?

What I tried:-

1. Tried convincing business to create a new custom task & event object. This is a problem as OOTB features are in use by many users.
2. Continue manual linking by users(entering linked activity details manually on custom field). This is core business problem which needs to be automated.

Links I referred(some of them):-

1. Can you create a Lookup to a Task?
2. Lookup Field on Custom Object related to Activity (Task) (aka create a related list on Activities/Tasks)

Answer 1

You can Enable Shared Activities in order to use the TaskRelation and EventRelation objects to link many custom object records to a single task or event, respectively. I haven't used this feature extensively, so you'll want to check out the documentation for this feature for more information on how it is meant to be used.