So I got this to work without too much trouble.
I did it as follows:
- created an Apex Sharing Reason on the Custom Object
- created a model on the __share object
- set conditions as shown.
- made the model create a row if one didn’t exist.
- using the action framework saved the model associated with the __share object
For those who want to understand this in more details, this page is useful.
When setting the condition for the RowCause, you must remember to use the Name of the apex sharing reason you created previously appended with __c.
