Fcitx: Setting Win + Space as "Trigger Input Method"?

I forgot the name, but the default IME for Gnome was something else than Fcitx. I chose Fcitx because its Japanese input (Mozc) provides the same Kana input as Windows’. Anyway, I want to use “Win + Space” as the shortcut to enable the IME, but I cannot do it. In Fcitx’s configuration, if I press Win+Space, it does not get it. If I press something else like Ctrl+Space, then it gets it, but I do not want to use Ctrl+Space because it is for IDE’s auto-completion. How can I set Win + Space for Fcitx?

I use ibus as input method, not Fcitx.
With ibus and m17n, gnome enable the shortcut you want. Is the package Fcitx-m17n already installed on your machine?
But, it depends also the input method you want to use, you can read here M17N - Fcitx

M17N also provides Cangjie , Hangul, and Pinyin, but it’s not a good solution for Chinese…

I found a similar question for Ubuntu, and it seems that the solution is go to Gnome Settings → Keyboard → View and Customise Shortcuts, and removing those Super+Space that are already assigned (search for “input”).