In lightning you are not able to do URL hacking. In order to do any prepopulation of fields, it is suggested to use a quick action.
If we have a related list object and we are trying to prepopulate the object of the related list, we would make a quick action and remove the "New" button. However, to a user, this will probably be very confusing since sometimes the new button is on the related list, sometimes you have to go to the top right/highlights panel to find the quick action button. Is there anyway to standardize both quick action and "new" button?
The link below mentions that it is not possible to put the quick action onto the related list.
I thought about the idea of overriding the "New" button to call a quick action, but that doesn't seem to be possible.
Any suggestions is greatly appreciated!