'I am an M.A in History from XYZ university' - Is it correct way to refer myself?

(OALD) postgraduate n. a person who already holds a first degree and who is doing advanced study or research; a graduate student

Postgraduate education, Degrees

"Postgraduate in History" is apparently not used (ngram). You do find "postgraduate in mathematics" although it is not used continuously (ngram), and "postgraduate in physics" (ngram). Nevertheless, using the term "postgraduate in History" seems justified, but on the condition that you keep on being a student; for instance while holding an MA you go on working towards a doctorate.

So, if you go on with your studies while working or trying to obtain some work you could say that you are a postgraduate in History with an MA; as well, "a postgraduate with an MA in History" would do. You could also say that you pursued your studies at the postgraduate level and were recently awarded your MA in History from XYZ University. Otherwise, that is, if you are finished with your studies, at least for the time being, you should not use "postgraduate".