in the Script Action Step for Things you will find the line var title = line.replace("->","")
add a new line below it with title = title.replace("- [ ]","")
then it should do what you want to achieve
The Script Step loops through all lines of the current draft and if the line includes “- [ ]” it will create a todo in Things. the title variable is then used as the title of the task. with the line i suggested above you are replacing the text “- [ ]” with an empty string.