I'm sure you resolved this issue since it's a year old, but since I can't find it documented anywhere I'll document how I did this.
- Go to system / task-scheduler / task
- Add a new task named UpdateAdblock, (DO NOT USE SPACES)
- Click Update List
- Go to system / task-scheduler / task / UpdateAdblock
- Enter your crontab-spec 17 2 * * * this will update daily at 2:17 am
- Go to system / task-scheduler / task / UpdateAdblock / executable
- Specifiy the full path of the executable /config/user-data/update-adblock-dnsmasq.sh (not the working directory)
- Preview and Apply
Using spaces in the task names on EdgeRouter X v1.9.0 generated an executable not found error for me, so I think there is a bug in the code.