Full Idea
Insofar as we have some insight into how God chooses, we can know a priori the laws of nature that God chooses for this best of all possible worlds. In this way, it is possible to have genuine a priori knowledge of contingent truths.
Gist of Idea
If we understand God and his choices, we have a priori knowledge of contingent truths
Source
report of Gottfried Leibniz (Conspectus libelli (book outline) [1678], A6.4.1998-9) by Daniel Garber - Leibniz:Body,Substance,Monad 6
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.246
A Reaction
I think it would be doubtful whether our knowledge of God's choosings would count as a priori. How do we discover them? Ah! We derive God from the ontological argument, and his choosings from the divine perfection implied thereby.