Can I know which part of the command is wrong and do I need to create any other things to have it permanent
If it worked by the terminal then it should work when hardcoded into the the config file.
Only... here in the given example it is evident that the add-in command is just misplaced. The init scripts in the /etc/init.d folder may have pointed to another *.conf file or, maybe, the script itself bears its own config variables. (I am no longer using Mint, there is no guaranty if I pretend to point to you which conf file.)
You may try use "find" or "grep" all over the /etc to locate the proper configuration file to your iface.
Hope this hint can help you.