Actually, the syntax error was something different, but when the form opened in a separate window, it was flagged on a statement that would have been OK in the grand scheme, just not in a standalone file.
Anyway, The program DOES compile in QB64 v1.1 with the old InForm but fails in QB64 v1.2 with the Beta 7 InForm. The compilation error log shows:
\AppData\Local\Temp\ccmOTGZh.o:qbx.cpp:(.text+0xac436): undefined reference to `MessageBoxA(int, char*, char*, int)'
\AppData\Local\Temp\ccmOTGZh.o:qbx.cpp:(.text+0xac5b5): undefined reference to `MessageBoxA(int, char*, char*, int)'
\AppData\Local\Temp\ccmOTGZh.o:qbx.cpp:(.text+0xac6e8): undefined reference to `MessageBoxA(int, char*, char*, int)'
/qb64/internal/c/c_compiler/bin/../lib/gcc/i686-w64-mingw32/4.7.1/../../../../i686-w64-mingw32/bin/ld.exe: \AppData\Local\Temp\ccmOTGZh.o: bad reloc address 0x10 in section `.data'
/qb64/internal/c/c_compiler/bin/../lib/gcc/i686-w64-mingw32/4.7.1/../../../../i686-w64-mingw32/bin/ld.exe: final link failed: Invalid operation
collect2.exe: error: ld returned 1 exit status
Perhaps that helps? FWIW, the program does invoke a messagebox for errors