What is your MachForm version?
Assuming you're using v18, you can edit machform/includes/helper-functions.php file.
Search this function mf_get_ssl_suffix() around line 1871.
At the end of that function (around line 1882), you should find this code:
return $ssl_suffix;
change it to become:
return "s";
That should force the generated links to use https.