The philosopher that coinded the concept of " a priori synthetic judgment" , that is, Kant, held the thesis that metaphysics proper does not contain such judgments.
According to Kant, there is no substantive possible knowledge regarding (1) the Soul ( the alledged thinking substance) (2) the World (3) or God.
But, with a more modest meaning of " metaphysics" one can talk about metaphysical a priori synthetic judgments.
Metaphysics, in the critical sense, is the theory that deals with the possibility conditions of experience. These conditions are a priori and are expressed in the " principles of pure understanding". The principle of these principles is that " the possibility conditions of experience are also the possibility conditions of the objects of experience".
You will find them in the Critique of Pure Reason :
(1) axioms of intuition
(2) anticipations of perception
(3) analogies of experience ( for example, the causality principle)
(4) postulates of empirical thought in general