If the 250 is the same as the 1050 you have to get a key from a Suzuki dealer, about £80, then have them code it to your bike, about £40, and once done the little red led on the ignition switch will flash three times when you turn the ignition on instead of two, to show three keys are coded to the bike.
If you are replacing a lost key, as opposed to getting a spare, I presume it’s the same, but a dealer will advise you.
You are responsible for having the key cut to fit the ignition switch at a local key cutters.
My dealer had awful problems coding the new key to the bike. They were brilliant trying but in the end it was likely that my Garmin satnav power box, fixed to the cowling frame was causing interference as once unplugged the key coded on the next attempt.