There is no function "fill_rectangle" but in Win32 SDK (or in MSDN) you can find

function:

int FillRect(
HDC hDC, // handle to device context 
CONST RECT *lprc, // pointer to structure with rectangle  
HBRUSH hbr // handle to brush 
);
May be Mr. Paul Curtis meant exactly this function?

And in unit "user32.f90" (user32.lib) you can find it declaration

interface !lib=user32.lib
integer(4) function  FillRect (hDC ,lprc ,hbr ) 
!DEC$ IF DEFINED(_X86_)
!DEC$ ATTRIBUTES STDCALL, ALIAS : '_FillRect@12' :: FillRect
!DEC$ ELSE
!DEC$ ATTRIBUTES STDCALL, ALIAS :  'FillRect'    :: FillRect
!DEC$ ENDIF
!DEC$ ATTRIBUTES REFERENCE :: lprc
use dfwinty
integer         hDC
type(T_RECT)    lprc
integer         hbr
end function FillRect
end interface