Thank you, @Robert Cemper !

This works like a charm! 

So, in my case, I also wanted to see what's in a stream object that comes into the method (you may ask me how I don't know this, as it is method I coded? ) I don't, as it is a generated one via %^REST):

ClassMethod submitForm(formData As %Stream.Object) As %Stream.Object

{

set formDataObj= {}.%FromJSON(formData.Read())

k ^SPOOL s %io=$I O 2 u 2 do formDataObj.%ToJSON() c 2 u %io

return $$$OK

And then I do a REST API call and can see data in the terminal with zw ^SPOOL global:

USER>zw ^SPOOL
^SPOOL(1,1)="{""amount"":0,""name"":""John Doe"",""taxid"":""AB123456C"",""nationality"":""british"",""email"":""john.doe@example.com""}"
^SPOOL(1,2147483647)="{67504,40535{2{"

Simple and easy! Fantastic, @Robert Cemper !