The syntax for compiling a help file (say myhelp.msg) into a binary help file (say myhelp.iem) is:
amkmessage myhelp.msg myhelp.iem
or
amkmessage myhelp.msg > myhelp.iem

Note that the syntax here is inconsistent with fcompile in that you must supply the name (including suffix) of the target binary file. This is consistent with Informix’s mkmessage program which has the same syntax.