This action is OS-specific actually. But some support is required from IDE side also. Please vote for https://youtrack.jetbrains.com/issue/IDEA-80673

The only solution I know, so far, is uninstalling and reinstalling the program.

This way during the installation windows faze, there will be a box you can tick so that the "Open folder as IDEA Project" is added automatically.

Just make sure when uninstalling to NOT choose to "delete" all the settings data etc.

You can use FileTypesMan to add custom context menu entries -- http://www.nirsoft.net/utils/file_types_manager.html
1) Find the "Directory" entry (usually near the bottom) and select it
2) Create new action (Ctrl+N):
- Action Name: IntelliJ IDEA
- Menu Caption: Open Folder as IDEA Project
- Command Line: "C:\Program Files (x86)\JetBrains\IntelliJ IDEA\bin\idea.exe" "%1" (or wherever you have it installed, you can also have it running and choose from running programs)

Thank you so much! This works like a charm ...

Thankkkk youuu so much!!! It helped me a lot ...

Thanks a lot

1. Locate the IntelliJ IDEA executable file (idea64.exe) on your system. By default, it can be found in the "bin" folder of your IntelliJ IDEA installation directory.

2. Right-click on the Windows Start button and select "Run" (or use the Windows Key + R shortcut).

3. In the Run dialog box, type "regedit" and press Enter to open the Registry Editor.

4. In the Registry Editor, navigate to the following path: HKEY_CLASSES_ROOT\Directory\Background\shell

5. Right-click on the "shell" key and select "New" -> "Key". Name the new key "Open with IntelliJ IDEA" (or any other name you prefer).

6. Right-click on the new key and select "New" -> "Key". Name the new key "command".

7. In the right-hand pane, double-click on the "(Default)" value and enter the following path: "C:\Program Files\JetBrains\IntelliJ IDEA 2021.3.2\bin\idea64.exe" "%V"

   Note: Replace the path with the actual path to the idea64.exe file on your system, and make sure to include the quotes around the path and the "%V" parameter.

8. Click "OK" to save the changes.

9. Close the Registry Editor.

If you don't want to install any applications, using the registry editor as suggested above is a great solution.
Here are a few extra steps you might want to do to set it up completely:

1. In the "Open as Intellij Idea" key, you can change the default value to change the actual message which appears.

2.In that same key, add a new string value named 'icon' with value: "C:\Program Files\JetBrains\IntelliJ IDEA 2021.3.2\bin\idea64.exe",0
- Replace the path with your actual path to idea64.exe
- This will add the intellij icon to the button.

3. Do all the steps from the previous and this comment but for the directory: HKEY_CLASSES_ROOT\Directory\shell (without \Background)
- this way the button will appear when you right-click a folder as well. Otherwise it only appears when you right-click the empty space inside a folder.
- A shortcut for this is to export the "Open With IntelliJ Idea" key (right click -> export). Then open it with notepad and edit the two paths by removing the '\Background part. Afterwards in regedit, click File -> Import and import it.

Hope that helps!

If you want to add "Open with IntelliJ IDEA" for single files just do all the steps from last 2 comments but for the directory: HKEY_CLASSES_ROOT\*\shell 

idea open with.reg

Windows Registry Editor Version 5.00
;folder context menu
[HKEY_CLASSES_ROOT\Directory\shell\open with idea]
[HKEY_CLASSES_ROOT\Directory\shell\open with idea\command]
@="\"C:\\Users\\User\\ideaIU-2020.1.windows\\bin\\idea64.exe\" \"%V\""
;file context menu
[HKEY_CLASSES_ROOT\*\shell\open with idea]
[HKEY_CLASSES_ROOT\*\shell\open with idea\command]
@="C:\\Users\\User\\ideaIU-2020.1.windows\\bin\\idea64.exe \"%1\""
; background context menu
[HKEY_CLASSES_ROOT\Directory\Background\shell\open with idea]
[HKEY_CLASSES_ROOT\Directory\Background\shell\open with idea\command]
@="\"C:\\Users\\User\\ideaIU-2020.1.windows\\bin\\idea64.exe\" \"%V\""

When installing Intellij with the standalone this registration is an option. Is this also supported when using the JetBrains ToolBox App?

Holger Brandl Yes. 

See also https://youtrack.jetbrains.com/issue/TBX-2540

This does not work in the same as the installer (without toolbox).  With the install, a menu entry was added to open directories as projcts in intellij. This has gone missing when using the toolbox app.
 

In in installer its this option which seems missing via toolbox:

Which fixes the problem (no toolbox used here)