Linking your Minecraft account to your Hypixel.net account will give you the "Creeperbook" Achievement and 15,000 Hypixel Network Experience.
If you do not have a Hypixel Forums account, please follow this guide to create one.
First, make sure you are logged into the Hypixel Forums on the account you wish to link to your Minecraft account.
Next, login to the Hypixel server and type in your chat "/linkaccount". Click where the command tells you to.
This will open a Hypixel.net link that will say you have successfully linked your Hypixel Forums account to your Minecraft account.
If you are not signed in, it will ask you to sign into the Hypixel Forums before linking the account for you.
To confirm you have linked the right Minecraft account or to unlink your Minecraft account, please visit https://hypixel.net/account/minecraft.