Get the message types https://github.com/sequenceplanner/ur_script_msgs, put them in your workspace with this repo then colcon build.
Then test the action:
ros2 action send_goal /ur_script ur_script_msgs/action/ExecuteScript '{script: "def code():\nmovej([1.0, 0.0, 0.0, 0.0, 0.0, 0.0],a=0.5,v=0.2)\nend\n"}'
You can use ctrl-c to cancel the goal request or leave it to let the goal succeed.