Predestined matters are things which human beings have no option at all in it e.g. your parents, your place of birth, your skin color, your height etc. 

Marriage is not predestined because it must have your own option and approval not be valid. Man and woman choose and decide whom they marry and accordingly they meet the results of their own decision.

You can pray to Allah (SWT) to make a specific person good for being your spouse. Allah (SWT) knows everything including the unseen and the future, and He grants whatever is good according to the reality and not necessarily as we think and imagine. Many things we wish and seek from Allah, can be harmful in the future, that is we should always submit to the Will, Wisdom and Mercy of Allah in seeking from Him. We should seek from Him to grant us whatever He knows good for us, not what we think good.

Wassalam