You can't. Assuming that you are talking about the HTTP GET and POST with the "Insert from URL" script step.
There are various plugins that allow you do your own HTTP GET and POST while specifying headers.
FYI if you're using the HTTP POST feature of the Insert from URL script step in FileMaker Pro v13 the headers are automatically set to:
I'd recommend a plugin like the BaseElements Plugin which allows you to explicitly set the headers:
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