To obtain a filer code, importers, brokers or carriers must submit a letter to the Port Director of the port where their entries are filed. The letter should be addressed to the Broker Management Officer. For those with operations at more than one port, the request should be made to the port where most of the business is conducted. 

For additional information, please visit the CBP website (cbp.gov): 
https://help.cbp.gov/s/article/Article-587?language=en_US