- Go to Sources > Snippets (it's in the left sidebar)
- Right click in the snippets window and select New
- Enter a file name and your desired snippet
- Right click on the snippet and select Run (or Ctrl/Cmd + Enter)
You'll also notice it has multi-line editing and confirmation for exit-without-saving.
Snippets source: https://github.com/bgrins/devtools-snippets