I want to install PyCharm, PhpStorm, etc. into some folder on drive D. Though JetBrains Toolbox not having the option to do so in my view.
So the question is that is it possible to customize the installation folder for JetBrains IDEs when installing via Toolbox?
Yes, you can configure the install location for apps in the Toolbox settings. However, this will install ALL apps under the specified directory (and move existing ones there). If you want to define a specific install location for each IDE, then that is not possible... yet.
Also see this support question. The second answer explains how you can manually update the install_location property in .settings.json. This will change the install location for future installations, and your existing apps will not be moved.