How to provide codes to reviewer without making it public
As others have mentioned, some venues offer an option of submitting additional material. 

If not, you have two options. 


Include your code as a single file into your submission in the PDF/A-3 format. (I do not recommend this option that much, though, since special software is required.)
Put your code on a public site as a password-protected archive. Most compressors such as zip and rar allow you to encrypt and compress.  Put the archive on a public site but give the password only along with the submission. E.g.


  Bibliography
  
  [S18] Smith, J., Example codes, 2018, http://www.example.com/example.zip, protected with the password example_password.