How to write a scan for PPO(12.26.9) line's slope (linear regression: 20)?

Here's a link that explains how to do it in general, including Slope:

https://support.stockcharts.com/doku.php?id=scans:advanced_scan_syntax:overlay_scans

So, in your case, the second term inside the Slope function parentheses would be PPO Line.

The crossing operator is "x". The first term always "crosses above" the second term. So if you want "crosses below", reverse the terms.

If you get stuck, post what you have and we will take a look.