Every slot on the tool bar has a hotkey. By default this is 1 through 0, but you may have changed them in the key bindings option.
If you want additional key bindings beyond that, you can post your suggestion for the game developers here: https://psteamcommunity.yuanyoumao.com/app/383120/discussions/1/

I mean, could I set up the ‘J’ key for flare, for example?

FDATerrorDragon/Sh4dow0fT3rror の投稿を引用：

I mean, could I set up the ‘J’ key for flare, for example?

Yes, you can do that now. I assume the key binds for the tool bar can be changed in your key bindings options menu,

ah thanks

I could be wrong, but I suspect they are looking to set up a key to directly launch flares without needing to switch away from the currently selected weapons to avoid the second or so delay it takes the game to switch to the newly selected item even when using the 1-9 keys.

Dyspayr の投稿を引用：

I could be wrong, but I suspect they are looking to set up a key to directly launch flares without needing to switch away from the currently selected weapons to avoid the second or so delay it takes the game to switch to the newly selected item even when using the 1-9 keys.

Ah yes, that would be a suggestion for Eleon then. When making a suggestion to Eleon, make sure to use vanilla examples whenever possible since they cannot support custom scenarios (and vice versa, hence why I cannot support this suggestion).

Dyspayr の投稿を引用：

I could be wrong, but I suspect they are looking to set up a key to directly launch flares without needing to switch away from the currently selected weapons to avoid the second or so delay it takes the game to switch to the newly selected item even when using the 1-9 keys.

ye swichting keys while fighting is not that useful at least at the Flare Launcher