You can install one of these:
https://cheapsprinklers.com/products/champion-dv-050-brass-1-2-drain-valves?variant=21948655370340&gad_source=1&gclid=EAIaIQobChMI5Oa72PjkgwMVJSfUAR2sCw09EAQYCSABEgL1CvD_BwEBuild a "basin" where your riser comes out of the ground that is filled with gravel/rock. This valve goes in that basin a foot or two below your sprinkler head (the rock gives it a place to drain into). When the system is on, this valve closes, and water sprays out of your sprinkler head. When the system shuts off, the pressure drops, and this valve opens, allowing any water above it to DRAIN, so that there's nothing in your riser or sprinkler head to freeze.
Have the line running from your system terminate at a T-connector in what will be that rock-filled "basin", pointing side/side. Put this valve on the side of the T, and put your riser/sprinkler head on the other side with a 90 degree..