The key parameter is for the authentication key, not the license key. Putting in a license key will result in the installation failing.
By leaving this field blank Symantec DLP will use the default authentication key. It is not recommended that you use this default. Although your installation will work, you will not be securely encrypting your data. If you do choose to use the default key during installation, you MUST change the key later so that it is unique. Be sure to choose the correct bit size for the authentication key as this can only be changed before installing the agent.