Add records to related list via API

Add records to related list via API

Hi!

I have a costume module called "Promotions" that has a related list to another module called "Buildings", the idea is that each promotion will be related to a few buildings (multi-select lookup field).
Usually I'm using the API for updating records - text, numbers, pick list, lookup etc..
I couldn't find how I can add or remove records from a related list. to be more specific - to add buildings to a specific promotion record.
The idea is to be able to update the related list with the API.

Any idea how to do it?
Thought about creating a function that will do it if there is no easy way.

Thanks
Oren