Linking collections though the API

So, I’m trying to link two entries through the API.
And I’m a bit lost
I have a parents collections and a comments collection.
In the parents collection I have a comments field with the type collectionlink
And in the commentscollection I have a parent filed with the type collectionlink
I can POST new entries in both collections, but I’m stuck when it comes to linking a comment to a parent.

Any suggestions on how to link them up?

Great product!