David Kimura PRO said over 2 years ago on Capturing Signatures with Signature Pad :
Check the resources section where I have the raw signature.js file linked. Also, around the 25 second mark, I explain the js file being moved to the vendors folder. If you're working on a more modern application with webpacker/esbuild/importmaps, I would probably use the npm package instead and would now use a stimulus controller to initialize the signature pad.