iPhone 4s asking for old Apple ID but nowhere to type code
You can type in that code immediately after your password in the password field.


  What if I use two-factor authentication on a device running older software? 
  
  If you use two-factor authentication with devices running
  older OS versions, you might be asked to add your six-digit
  verification code to the end of your password when signing in. Get
  your verification code from a trusted device running iOS 9 and later
  or OS X El Capitan and later, or have it sent to your trusted phone
  number. Then type your password followed by the six-digit verification
  code directly into the password field.


Source: https://support.apple.com/en-us/HT204915