Hi there! This is a rejected suggestion - the Scratch Team aren't going to add it. Here's why:
1.2 “When stop sign clicked” hat block
The block below would allow users to click the stop sign to run a script. However, the stop sign is designed to stop all the scripts in the project. With this block in place, more scripts will start when you want the project to stop, thus defeating the purpose of the stop sign. This could also be rather confusing for the “stop all” block. Regardless, there is a workaround that uses the “when (timer) > (number)” hat block; one such workaround can be found in this post.
when stop sign clicked :: events :: hat