Timing of Warranty Transfer Purchase

I purchased a used Flex that has a portion of the warranty remaining. I know that a Warranty Transfer can be purchased if I wish to have the remaining portion of the Warranty transferred to me as the new owner. However, I cannot find anywhere the details on when the Warranty Transfer needs to be purchased? Is it at the time of sale to the new owner or can it be anytime up to the end of the original warranty period. Could it even be immediately before sending the used Flex Radio back to Flex for repairs if it malfunctions before the end of the original warranty period?