The object A is related in a 1:M relationship with the object B: I have to create, with an action, one or more objects B under the object A. I know how to create the object B, but I am not able to create the relationship between the two objects.
Basically you need to populate the relationship Id with the Id of the record A.
When you create a relationship between 2 Custom Objects, which I presume this A and B are, the relationship attribute that needs to be populate is of this format [ObjectName]_Id_[RelationshipName].