When clicking the ‘new’ button, make sure to call a microflow instead of going directly to the pop-up page.
In this microflow you can retrieve the latest object of EntityA. You can do this by retrieving a list of EntityA objects and sort this list descending based on createDate (is you use this system variable) or use another attribute to logically sort on (for example any ID or timestamp attribute will do). Then use the list operation head to get the top object (being the last one).
Then create a new object of EntityA, adding the attributes you already want to fill and use the show page action to navigate to your pop-up page, adding the NewEntityA object as parameter.
There is no automatic way to do this. You would have to write a microflow to fulfil this requirement. Hope this helps!