format PE GUI 

    include 'win32ax.inc' 

    entry start 

    section '.idata' import data readable writeable 

    library kernel32,'KERNEL32.DLL' ,\ 
            user32,'USER32.DLL',\ 
            advapi32,'ADVAPI32.DLL',\ 
            shell32,'SHELL32.DLL' 

    include '\fasmw17002\include\api\kernel32.inc' 
    include '\fasmw17002\include\api\user32.inc' 
    include '\fasmw17002\include\api\advapi32.inc' 
    include '\fasmw17002\include\api\shell32.inc' 

    struct PROCESSENTRY32 

    dwSize                     dd ?  
    cntUsage                   dd ?  
    th32ProcessID              dd ?  
    th32DefaultHeapID          dd ?  
    th32ModuleID               dd ?  
    cntThreads                 dd ?  
    th32ParentProcessID        dd ?  
    pcPriClassBase             dd ?  
    dwFlags                    dd ?  
    szExeFile db MAX_PATH dup(?) 

    ends 

    target db 'notepad.exe',0 ; notepad as example
    szDLLName   db "Project1.dll", 0
    sizeof.szDLLName = $ - szDLLName - 1
    dwThreadID  dd ?
    threadHandle dd 0
    pentry32 PROCESSENTRY32 
    TH32CS_SNAPPROCESS = 2 
    snap dd 0 
    id  dd 0 
    kernel32dll TCHAR 'KERNEL32.DLL',0
    LoadLibFunc TCHAR 'LoadLibraryA',0

    section '.code' code readable executable 

proc InjectDll,Pid:dword

local   ProcAddress:DWORD,Alloc:DWORD,Open:DWORD

    invoke  GetModuleHandle,kernel32dll
    invoke  GetProcAddress,eax,LoadLibFunc
    mov [ProcAddress],eax

  invoke OpenProcess,PROCESS_CREATE_THREAD+PROCESS_VM_OPERATION+PROCESS_VM_READ+PROCESS_VM_WRITE+PROCESS_QUERY_INFORMATION,0,dword[Pid]
   mov [Open],eax
  test eax,eax
  je InjectDll_exit

    invoke VirtualAllocEx,eax,0,dword[sizeof.szDLLName],MEM_COMMIT,PAGE_READWRITE
    mov [Alloc],eax
    invoke WriteProcessMemory,,dword[Pid],eax,szDLLName,dword[sizeof.szDLLName], NULL
    invoke CreateRemoteThread,dword[Pid],0,0,dword[ProcAddress],dword[Alloc],0,dwThreadID
     mov     [threadHandle],eax
    invoke WaitForSingleObject,dword[threadHandle],0xFFFFFFFF
    invoke CloseHandle,dword[threadHandle]
    invoke CloseHandle,dword[Open]
    invoke VirtualFree,dword[Alloc], 0, MEM_RELEASE
    je InjectDll_exit
  InjectDll_exit:
  ret
endp

    start: 

    mov    [pentry32.dwSize],sizeof.PROCESSENTRY32 
            invoke CreateToolhelp32Snapshot, TH32CS_SNAPPROCESS, 0 
    mov    [snap], eax 
            invoke Process32First,[snap],pentry32 
    .loop: 
            mov    edi,target 
            invoke StrStrI,pentry32.szExeFile, edi 
            cmp    eax,0 
            je     .next 
            call   .pid 
    .next: 
            invoke Process32Next,[snap],pentry32 
            cmp    eax,0 
            jne    .loop 
    .pid: 

    mov dword[id],pentry32.th32ProcessID 

    invoke MessageBox, NULL,id,0,0 
    stdcall InjectDll,id

    jne .bye 

    .bye: 

    invoke ExitProcess,NULL
    

According to "Microsoft", second parameter must be a pointer to a null terminated string. "id" probably is not, since it was returned as an ordinary integer by other functions.

You will have to make a string out of "id" first, pick the pointer to it and then pass it as a parameter to "MessageBox". On Windows, you can do it by calling "wsprintf", I think.