If you can modify the form, specify a different action target to a host/cgi that you own to log the submitted data.
If the form doesn't have the protocol/host specified in the action, you might be able to get away with a simple ssl proxy, using stunnel or the like.
You might be able to perform some DNS trickery after the getting the form, but before submitting it and direct the app to a web server you own with a CGI script that logs all the input variables.
If you can't do any of the above and the app you are using uses the standard win32 networking/crypto APIs, you can hook the SSL calls in wininet. There are products available to do that such as this one.
I only suggest the crypto hooking last since it costs money, the rest of the options cost time.