Manga with isekai working mc

Is it: Shiro no Koukoku Monogatari

Alt title: Legend of the White Kingdom

Having lost his money, lover, and future, a man who has given up on everything finds himself reincarnated in the kingdom of Altodestenia. He, Lectival, has no memories prior to being saved by Meriela, the daughter of an aristocrat, and now finds himself rising up to save the kingdom in its current crisis.

He has to accept the title of King and needs to lead an army into battle.

Or is it: How a Realist Hero Rebuilt the Kingdom

Alt title: Genjitsu Shugi Yuusha no Oukoku Saikenki

"O, Hero!" With that cliched line, Kazuya Souma found himself summoned to another world and his adventure--did not begin. After he presents his plan to strengthen the country economically and militarily, the king cedes the throne to him and Souma finds himself saddled with ruling the nation! What's more, he's betrothed to the king's daughter now...?! In order to get the country back on its feet, Souma calls the wise, the talented, and the gifted to his side. Five people gather before the newly crowned Souma. Just what are the many talents and abilities they possess...?! What path will his outlook as a realist take Souma and the people of his country down?

He has black hair but has the ability to control three pens and needs to prove himself. He also gets summoned as a replacement for king.