What happens if you simply don't specify any field in the script step? FM will probably take the field that is currently active.
Why do you need to copy the data to the clipboard? There can be good reason for that, but there are also many "not good" reasons for it as well...
You might "generalize" such a copy action by:
Go to Object
Set Field [Globals::GlobalText field ; Get ( ActiveFieldContents) ]
Copy [Globals::GlobalTextField ]
This way you are always copying from the same field, but only after using the set field step to load this one field with the data from any number of other fields that may need to be copied.