If you're unable to find the designer.exe file, you can locate it here:

C:\Users\<user>\PycharmProjects\<project>\.venv\Lib\site-packages\qt5_applications\Qt\bin\designer.exe

This path is valid for setups using Python 3.9 with a virtual environment in PyCharm. Replace with your Windows username and with your project's name.

I wanted to add this as a comment to Joker's answer, but since commenting requires 50 reputation, I'm posting it as an answer instead.