How do I retrieve my iPhone passcode?

You can't retrieve it but you can use the following procedure: If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Follow this Link

If you forgot the passcode on your iPhone, or your iPhone is disabled

If you forgot the passcode on your iPhone, or if a message says that your iPhone is disabled, learn what to do.

https://support.apple.com/en-ca/HT204306