I do not have that exact tractor - but the B7100 is somewhat similar - and to my knowledge there is no way to "lock" it up - short of some type of external strapping. I had to "wire" mine up one time to fix something - but to my knowledge there is no mechanical way to do it.
Seems to me there are two little metal strips (one on each side of the frame) that swing down to clip onto the deck. Been a long time, but they were about 1" wide and 3" long.