A sold-to ID is assigned to a purchasing RS Components customer. If you don't have one, try any of these 2 options:
1. Enter a dummy value (should be 8 digits) like 12345678
2. Enter 10 zeroes into the sold-to id field and then delete them. The Register/Update button should be enabled for you to complete the process.
Note that going forward, 'sold-to id' is no longer a requirement for DesignSpark registrations. Only older software which have hardcoded fields will be patched with a simpler licencing system. This is now complete for most of our desktop software except Mechanical (will be shortly rolled out in October).