Hi Chris, Welcome to our forum!
A 64 bit program and a 64bit DLL to go with it:
%filename "TestHello164.exe"
include "..\..\inc\RTL64.inc"
declare function Hello lib "t164.dll" (s as string) as string
print Hello "World!"
%dll
%filename "t164.dll"
#include "$\inc\RTL64.inc"
'print "t164.dll Loaded"
function Hello(s as string) as string, export
function = "Hello "+s
end function