There is a guide to create workflow, but honestly not so helpful.
https://www.jetbrains.com/help/youtrack/standalone/Workflow-Guide.html
It creates issue entity but no sprint. I can't find it, how can I add properties (date-range, name, board) to the new sprint.
var newSprintEntity = new entities.Sprint(/* ??? */);
Is there any solution for this?
YouTrack workflows were not intended to modify project configuration, so there was no way to create a sprint from inside the workflow script until recently.
With https://youtrack.jetbrains.com/issue/JT-11968 implemented is should be possible to create new values for a custom field. If your sprints are custom field based, then any new field value will produce a new sprint.
Please note, that https://youtrack.jetbrains.com/issue/JT-11968 has not been released yet.