The values don't have to be in 'hex' - you have to create a string whose characters are identical to the binary image of the structure.

If we disregard FoxPro/Mac for a moment then we can assume that we're talking Little Endian here (i.e. x86 CPUs and compatibles). This means that values a arranged in memory with the least significant byte at the lowest address and the most significant byte at the highest. For example, given the following function definitions:

#define OxFF  255

* pack a word
function pw (v)
   return chr(bitand(m.v, OxFF)) + chr(bitand(bitrshift(m.v, 8), OxFF))

* pack a long 
function pl (v)
   return pw(m.v) + pw(bitrshift(m.v, 16))

then you could build a 'structure' containing two 32-bit integers like this:

local cStruc
cStruct = pl(m.tnField1) + pl(m.tnField2)
= ApiFunc(@m.cStruc)

(Note: I wrote the functions when I had my 'brief identifiers' period some five years ago and because they work and because bit-twiddling heavy code actually benefits from having the short identifiers for trivia like pack/unpack I have have not changed them *g*)

If you want to skip the gory details, head on over to the Fox Wiki and read the ApiStructureClass topic. The class in question will handle the bit twiddling for you.