If am not wrong, you are trying to set data from former object to New object... So in your case it should be parent and child objects...
If yes, then you can set in 2 ways...
1. Context : In XML by using context you set the value, or access the context value in your Child Object defaults method in Py Class..
2. Parent: In XML, by using the parent keyword also you can set value of its parent data...
(Ex. <field name="O2M_lines" context={'default_CHILD_FIELD' : parent.PARENT_FIELD })
Tips: use Ctrl + H, if you are using studio, to perform search action in the entire standard addons module for more examples