I found a tutorial on the site of MQL5.com to create GUIs for your MT5 indicators
you know how it is a pain in the butt to optimize a strategy manually ? reopening the indicator window then changing a setting then closing, then analysing the results, etc
it's painfully slow.
but now with this code you can create a GUI window with any parameter you want, and it can be updated with any change you do to a parameter
then the indicator is changed in real time

here is an example with multi-osc
GUI is dynamically updated ! this is powerful

link to full tuto:
https://www.mql5.com/en/articles/2788