invoke  GetDlgItemText,[hwnddlg],IDC_EDIT1,name,16 
mov ecx,eax
        mov ebx,ecx
        xor esi,esi
  loop0:
        dec ecx
        cmp ecx,0
        jnz loop0
  loop1:
        mov al,[string+ecx+esi]
        cmp ecx,ebx
        ja loop0
        inc esi
        cmp esi,17
        jnz loop1   
    

I just want to do a string with 16 chars and to put for example: AA in the lenght of this string his 16.

 format PE GUI 4.0
entry start

include 'win32a.inc'

section '.data' data readable writable

   IDC_EDIT1  = 1000

   name            db 40
   strtemp         db 16,0
   serial          db 17,0

   StrToGen        db '_r <()<1-Z2[l5,^',0

section '.code' code readable executable

  start:

        invoke  GetModuleHandle,0
        invoke  DialogBoxParam,eax,101,HWND_DESKTOP,DialogProc,0
        or      eax,eax
        jz      exit

  exit:
        invoke  ExitProcess,0

proc DialogProc hwnddlg,msg,wparam,lparam
        push    ebx esi edi
        cmp     [msg],WM_INITDIALOG
        je      wminitdialog
        cmp     [msg],WM_COMMAND
        je      wmcommand
        cmp     [msg],WM_CLOSE
        je      wmclose
        xor     eax,eax
        jmp     finish
  wminitdialog:

         jmp processed
  wmcommand:
        cmp     [wparam],BN_CLICKED shl 16 + IDCANCEL
        je      wmclose
        cmp     [wparam],BN_CLICKED shl 16 + IDOK
        jne     processed
        invoke  GetDlgItemText,[hwnddlg],IDC_EDIT1,name,16
        mov ecx,eax
        mov ebx,ecx
        xor esi,esi
  loop0:
        dec ecx
        cmp ecx,0
        jnz loop0

  loop1:
        mov al,[name+ecx]
        mov byte [strtemp+esi],al
        cmp ecx,ebx
        inc ecx
        ja loop0
        inc esi
        cmp esi,17
        jnz loop1
        xor eax,eax
        xor ecx,ecx
        xor ebx,ebx
        xor esi,esi
        xor edx,edx
        mov ebx,19h
  loopsr:
        mov al,[strtemp+ecx]
        mov dl,[StrToGen+ecx]    ; doesn't move strtogen it moves strtemp.
        xor al,dl

        IDIV ebx
        add al,41
        mov [serial+ecx],al
        inc ecx
        cmp ecx,17
        jnz loopsr

        invoke MessageBox,HWND_DESKTOP,serial,serial,MB_OK
        jmp processed

  wmclose:
        invoke  EndDialog,[hwnddlg],0
  processed:
        mov     eax,1
  finish:
        pop     edi esi ebx
        ret
endp

section '.idata' import data readable writeable

  library kernel,'KERNEL32.DLL',\
          user,'USER32.DLL'

  import kernel,\
         GetModuleHandle,'GetModuleHandleA',\
         ExitProcess,'ExitProcess',\
         CreateFile,'CreateFileA',\
         WriteFile,'WriteFile',\
         CloseHandle,'CloseHandle'
  import user,\
         DialogBoxParam,'DialogBoxParamA',\
         EndDialog,'EndDialog',\
         GetDlgItemText,'GetDlgItemTextA',\
         MessageBox,'MessageBoxA'

section '.rsrc' resource from 'C:\fasmw165\projects\falcon1\dlg.res' data readable     

invoke  GetDlgItemText,[hwnddlg],IDC_EDIT1,name,16
MOV EDI, EAX     ; result of function call in EDI & EAX
XOR ECX, ECX    ; set ECX & ESI to zero
XOR ESI, ESI
Loop1:  ;FIND THE END OF RETUNED STRING
        CMP BYTE [ECX+EDI], 0h
        JE  Loop2 ; if('ret_str'[ecx] == 0) goto Loop2
        INC ECX   ; ECX++ // increase index
        JMP Loop1 ;repeat

Loop2:  ;ADD OTHER STRING TO STRING RETURNED BY GetDlgItemText
        CMP ECX, 10h        ;IF LETTERS COUNT IS EQUAL 16?
        JE  End_Loop2       ; goto End_Loop2
        MOV AL, BYTE [String+ESI] ;else AL = String[ESI];
        MOV BYTE [ECX+EDI], AL    ;ret_str[EDI] = AL
        INC ESI         ;ESI++ //increase index of String
        INC ECX         ;ECX++ //increase index of 'ret_str'
        JMP Loop2       ; repeat
End_Loop2:              ; Finished