Following an unpleasant experience with a simple 64-bit example that built fine but ran with mysterious WinAPI errors (http://masm32.com/board/index.php?topic=6046.msg64038#msg64038), I've revised the handling of manifests in MasmBasic version 3 March 2017 (http://masm32.com/board/index.php?topic=94.0). Here is an example that shows if the manifest works, using the ComCtl32$() macro:
include \Masm32\MasmBasic\Res\JBasic.inc ; part of MasmBasic (http://masm32.com/board/index.php?topic=94.0)
Init ; OPT_64 1 ; put 0 for 32 bit, 1 for 64 bit assembly
MsgBox 0, ComCtl32$("Uses ComCtr32.dll version %i.%i\ncheck your manifest"), Chr$("Built with ", @AsmUsed$(1), " in ", jbit$, "-bit format"), MB_OK
EndOfCode
Project attached. The screenshot shows the MessageBox generated with the standard MasmBasic manifest, demonstrating that version 6.16 of the common controls is being used. If it shows the old 5.82, there is something wrong with the manifest :(