About Japanese input using Ubuntu US keyboard

Ubuntu 18.04.3 LTS uses fcitx as its input method.

Currently, we are switching between Japanese input using the following settings: Left alt key to turn off the input method and Right alt key to turn on the input method.

For example, if you are using an application such as vscode and you press alt to switch between input methods.
Then the cursor will be taken toward the menu bar, and you will not be able to enter characters.

Attempted to set the key configuration in vscode, but could not find the configuration for alt alone.

Please tell me how to avoid taking the cursor to the menu bar while switching inputs with the left and right alt keys.