If I understand what you are asking correctly, You click the Blue Wi-Fi button at the top right corner of the post of the person who's friend code you want. (you can also do this on their profile) It sends a request to that user. If the user accepts, it adds the friend code to your registry.
You cannot post your friend code or ask someone else for theirs without using this.
Signature-------------- When the moon found the sun, he looked like he was barely hanging on But her eyes saved his life, in the middle of summer <3