Attaching Sigma 18-50mm lens to XT-5

Hi folks,

Terribly sorry if this is painfully obvious. On the recommendation of a friend I've just received a XT-5 camera with an (unattached) Sigma 18-50mm lens. I've tried attaching it to the camera but seems slightly too big for it to slot into place. The lens doesn't have a little red dot - like other lens - to indicate where to slot it into the camera.

Thanks in advance,

Brian