According to the publisher the code for the demo will be included in only the first run of retail copies of Type-0 HD and bundled with digital copies for "a limited time only."
You'll know if your copy has a demo included as the retail copies of Type-0 HD will be branded as 'Day One' versions.
Codes must be redeemed by March 20, 2016 to redeem the download codes for the Final Fantasy 15 demo. The demo itself will be playable on the day Type-0 HD releases and will allow for both Japanese and English voice-overs, as will Type-0 HD.
Apparently the Episode Duscae demo will offer “several hours” of playtime and plenty of secrets too.
You can pre-order below.