You need to create a Entity A and Entity B with 1 to * Association, Then Create a Entity B object and set the Entity A Association and refresh in Add button, In the list view retrieve Entity B by ByAssociation , Then Finally commit the commit the Entity A and Entity B in save button
As per my assumption you need to create a object while clicking + button, For that create a microflow in that create the object which you want to display and commit the object then call the microflow in the + button