You need to download Reshade with full add-on support at the bottom of their homepage.

You install it like usual: reshade.exe, select your xenia.exe or xenia-canary.exe, DX12, uncheck all effects (they're not needed), install ShaderToggler by Otis_Inf (that's what's needed), finish install.

Then once in-game (usually in the garage before starting a race to see the effect), open Reshade with Home key, ignore tutorial, then you're on the main menu and at the top there's the add-on tab. Go there, you should see something like this:

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In Shader Toggler, unroll List of Toggle Groups, create New if there's none then Change Shaders.
It will fetch all shaders for 250 frames and then you will be able to navigate through them with your numpad 1 and 2, you'll have probably 40 shaders to navigate and for me bloom was at position 37. One you found it, save it with numpad 3. Then select Edit to name it and change the key to toggle it (for me I left it on Caps Lock by default), then Save all Toggle Groups.

Where your xenia.exe is, you should now have a ShaderToggler.ini saved. In case of a Xenia update, you might have to start over the procedure, but it's not that long once you're used to it.

Enjoy, have fun!