I believe the main use in modern Japanese is in the fixed expression あるまじき, which generally takes either -に or -として with the preceding noun. I think まじき’s modern use with verbs is extremely limited, having been largely replaced by まい, which itself derives from the same archaic Japanese auxiliary verb まじ. Of historical note, まじき is the attributive form (連体形) of まじ, so when you see a word like this ending in -き, it will always be modifying something coming after it. Compare, for example, the difference between ごとき (attributive) and ごとし (final). Classical Japanese had separate sentence-ending (終止形) and attributive (連体形) forms that have merged in modern Japanese, but you can still see the difference in certain anachronisms that have survived into the modern language.