I got the impression that SUBMIT:ing a form will automatically pass on the variables used in the original form to the target item. Your impression is correct. However my experiments doesn't seem to do that. What happens? What are your experiments.
The parameter passing is an essential task of ZPubllisher, more precisely its "mapply" function. It works very reliable.
Probably, your experiments are slightly wrong.
Is there another way of doing this than the URL?para=something ? Submitting a form with method="GET" (the default) is equivalent to a "URL?c1=v1&c2=v2&...".
You may want to look either at "REQUEST" (display it in the target (action) page) or in "var/Z2.log" (the ZServer Log file). Experiment with "method=GET" (until everything works as expected) because,then, you see the parameters in the ZServer log file.
Yes, showing REQUEST in the target did indeed solve the problem: It was Formulator not using the name of the field only as a variable name, but a name of field_<name>. Now this finally seems to work as I intended. Thanks to you all for your hints, it helped a lot to know where to look. Dag