Hi,
Answering my own question: The GET/POST method only appears for the "End" field. It's deactivated for "Start" and "In Between". However the value is stored in the database in field "options2" and defaults to "GET". To have it be "POST" you can first configure the field" as "End", change the value to 'POST" , save and then change the type of field to "Start" or "In Between". Not sure it matters though. In my tests as long as the "End" field is "POST" it seemed to work and I was able to save all values in my long forms. (whereas with "GET" some values were not saved).
Regards.
Xav.