At this point you can only do it if you have a "Certificado Digital" which you need to request online, identify yourself personally in certain governmental offices and then download it to your computer. Once you have it, you can create your account in SES.HOSPEDAJES which is, as you say, the new platform for reporting.
I have enquired to the Ministry of Interior because the law indicates that non-professional user CAN register and report offline, but I believe nobody put together a process to do so.
This was the less thought through law they could come up with. Written by some clerks that never booked a holiday for themselves or paid online before... just good-old cash or physical credit card at a hotel reception or travel agency.
SES.HOSPEDAJES includes the possibility to indicate that the reservation was paid at "PLATFORM" but the law does not indicate that possibility... quick and cheap fix 😉