Searching for "cracks" or "free keys" on third-party sites often leads to: Security Risks

Maya was a freelance graphic designer on a tight budget. She needed to resize and watermark 200 product photos for a client by morning. She had downloaded the free trial of PhotoPad Image Editor by NCH Software, but the trial added a small watermark to every export. The full license cost $39.95 – a fair price, but more than she wanted to spend right now.

Instead of looking for a serial key, I'd recommend you try the following:

If you have purchased a license and your serial key is not working: PhotoPad Photo Editor Express Edition - Microsoft Store