format PE GUI 4.0
include '%fasminc%\win32a.inc'
start:
invoke MessageBox,0,msg,ttl,0
ret

msg db 'Hi!',0
ttl db 'FASM is fun!',0

data import
library user32,'user32.dll'
import user32,\
MessageBox,'MessageBoxA'
end data
    

proc DlgProc,hWnd,uMsg,wParam,lParam
        enter   
        push edi esi ebx
        
        mov eax,[uMsg]
        cmp     eax,WM_COMMAND
        je      jCOMMAND
        cmp     eax,WM_INITDIALOG
        je      jINITDIALOG
        cmp     eax,WM_CLOSE
        je      jCLOSE
        xor eax,eax
        jmp finish

jINITDIALOG:
        mov eax,[hWnd]
        mov [gWnd],eax
        
        mov eax,1
        jmp finish
        
jCOMMAND:
        mov eax,[wParam]
        cmp     eax,1000
        je      a1000
        xor eax,eax
        jmp finish
        
a1000:
        invoke SendMessage,[hWnd],WM_CLOSE,0,0
        mov eax,1
        jmp finish
        
jCLOSE: 
        invoke EndDialog,[hWnd],0
        mov eax,1
        
finish:
        pop ebx esi edi
        return
endp
    

include '%fasminc%\macro\if.inc'    

.if eax,e,4 ; if eax equal to 4
; some code
.elseif eax,ge,7 ; if eax greater or equal to 5
; some code
.elseif ebx ; if ebx is not 0
; some code
.endif

.while [var],b,1000 ; while [var] is below 1000
; some code
.endw

.repeat
; some code
.until eax ; until eax is not 0    

But I think people usually change from FASM to MASM32 because of the HLL macros like .IF, .WHILE, etc...

I think FASM should come with pre-built in HLL macros.

it's a pain to take care of all those cmp's and je's. It is handy with macros like .IF and .SWITCH.
I think you know what I mean.

format PE GUI 4.0
include '%fasminc%\win32a.inc'
include '%fasminc%\macro\if.inc'
start:

invoke MessageBox,0,msg,ttl,MB_YESNO
.if eax,e,IDYES
    invoke MessageBox,0,answer,ttl,0
.else
    mov esi,answer
    .while byte[esi],ne,'Y'
       inc esi
    .endw
    mov word[esi],'No'
    add esi,2
    mov byte[esi],0
    invoke MessageBox,0,answer,ttl,0
.endif
ret

msg db 'Yes or no?',0
ttl db 'HLL macros test with fasm',0
answer db 'Answer: Yes',0

data import
library user32,'user32'
include '%fasminc%\apia\user32.inc'
end data
    

for a=1 to 10
{
  for b=1 to 10
  {
    ... <part1>
    if something
      goto OutOfLoop
    ... <part2>
  }
}
OutOfLoop:
    

OzzY: By the time, you will learn to code not structured code (with .if, .while), but what i call "linear code", where you have (kind-of) independent blocks of code linked with jumps.