Not sure what you're asking...?
A bounceback promotion is only available for booking before you check out of a Disney resort. Typically there is an in-room flier listing the promotion, the codes, and the number to call to reserve. If there is no flier you can visit the main desk and ask if there is a bounceback promotion.
From what I understand there is no option to reserve a bounceback online, and even if there is, I assume it would still need to be booked before you check out.
Edit: Okay now I think I get it. You're asking, if you check-in online for your trip, do you then have immediate access to whatever the current bounceback promotion is. Pretty sure the answer is no...but that would be pretty interesting to try if you already had the BB details ahead of time (phone number and code).