Taskbar show/hide app icon example:

taskbar.asm

format PE64 GUI 6.2 at 0x1234_56780000
include 'mywin.inc'

GWL_EXSTYLE := -20

TaskbarDemo:
	iterate msg, WM_INITDIALOG,WM_COMMAND,WM_CLOSE
		cmp edx,msg
		jz .msg
	end iterate
	xor eax,eax
	retn

label .WM_INITDIALOG at .message_processed

.WM_CLOSE:
	enter 32,0
	EndDialog rcx,0
.message_processed_leave:
	leave
.message_processed:
	push 1
	pop rax
	retn

.WM_COMMAND:
	iterate cmd, ID_EXIT,ID_SHOW,ID_HIDE
		cmp r8d,BN_CLICKED shl 16 + cmd
		jz .CMD_#cmd
	end iterate
	jmp .message_processed

label .CMD_ID_EXIT at .WM_CLOSE

.CMD_ID_SHOW:
	enter 32,0
	SetWindowLongA rcx,GWL_EXSTYLE,WS_EX_APPWINDOW
	jmp .message_processed_leave

.CMD_ID_HIDE:
	enter 32,0
	SetWindowLongA rcx,GWL_EXSTYLE,WS_EX_TOOLWINDOW
	jmp .message_processed_leave



WinMain:entry $
	virtual at RBP-.FRAME
			rq 4
		.P5	dq ?
		_align 16
	.FRAME := $ - $$
	end virtual
	enter .FRAME,0
	GetModuleHandleA 0
	xchg rcx,rax
	DialogBoxParamA rcx,IDD_TBDEMO,HWND_DESKTOP,ADDR TaskbarDemo,0
	ExitProcess 0
	int3

.end _winx.entry ; _winx.end? _end ; generate import section

section '.rsrc' resource data readable

ID_EXIT := IDCANCEL
ID_SHOW := 100
ID_HIDE := 101

IDD_TBDEMO := 1

directory RT_DIALOG,dialogs

resource dialogs,\
	IDD_TBDEMO,LANG_ENGLISH+SUBLANG_DEFAULT,tbdemo

dialog tbdemo,'Taskbar item control',70,70,170,24,WS_CAPTION+WS_POPUP+WS_SYSMENU+DS_MODALFRAME
	dialogitem 'BUTTON','Show',ID_SHOW,4,4,45,15,WS_VISIBLE+WS_TABSTOP
	dialogitem 'BUTTON','Hide',ID_HIDE,54,4,45,15,WS_VISIBLE+WS_TABSTOP
	dialogitem 'BUTTON','Exit',ID_EXIT,120,4,45,15,WS_VISIBLE+WS_TABSTOP
enddialog

The other method uses ITaskBarList COM interface:
https://github.com/tgrysztar/fasmg/blob/master/packages/x86/examples/windows/usecom/usecom.asm