On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough for Take Me a Husband side quest.
Reward for completing the mission: 1,200 XP, 40 silver.
In the place shown on the picture above, you will meet a woman who is called "old maid" by her children. From your conversation with her, you will learn that she is pining for the man with whom she once swore love. Unfortunately, his father won't let him get married so you must kidnap him and bring him to the old maid.
The man's house is not far away, on a hill - there are very many red flowers around. When you talk to him and tell him about the fake kidnapping, the man will not know what is going on and a fight will ensue.
Defeat the man in battle, then throw him on your back and carry him to the woman. After a short cut-scene, the mission will end.