It's possible to set this via AutoHotkey, but it's not the best solution. Requires the window to be maximized. Save the file as e.g. SiYuan_show_keymap_hotkey.ahk and run on startup if wish.

#Requires AutoHotkey >=2-
#SingleInstance Force

#HotIf WinActive("ahk_exe SiYuan.exe")  ; The hotkeys following this line are only active when SiYuan.exe is the active window.
; Press Alt+K to show SiYuan keymap (requires this plugin: https://github.com/zuoez02/siyuan-plugin-keymap)
!k::{
	; The coordinates for the "Show keymap" button in the plugin, assuming:
	;   - SiYuan.exe is fullscreen
	;   - and active on the main monitor
	;   - and the window is raised up from the bottom of the screen by whatever the width of Windows taskbar is
	clickX := 1872
	clickY := 1017
    MouseClick("left", clickX, clickY)
}
#HotIf