The query string is not natively available as a token.
You can use this method (with the limitations noted) to pass the query string as part of the {{trigger.web page}} token. But this still isn't parsing the query string (i.e. you will not see the utm_content value alone).
For parsing on the server side, you need a webhook -- this is something we do a lot, like when an outside agency sets up rather opaque UTM params and we want to translate them into friendly names back on the lead.
But parsing is only necessary when you want to post data without a form. As you are using a form, I'm not convinced you can't do this using standard hidden UTM fields and spare yourself the complexity of server-side parsing. Why not add the field to the form, but just empty the value (using Forms JS) if it isn't "Promo1" (or any other interesting value):
MktoForms2.whenReady(function(form){
// list your interesting values for certain fields
// non-interesting values will be removed
var interestingValues = {
"utm_content" : ["Promo1","another value"]
};
/* --- NO NEED TO EDIT BELOW THIS LINE! --- */
form.onSubmit(function(form){
var currentValues = form.getValues();
Object
.keys(interestingValues)
.filter(function(field){
return !interestingValues[field].some(function(value){
return value === currentValues[field];
});
})
.forEach(function(field){
var mktoFormData = {};
mktoFormData[field] = "";
form.setValues(mktoFormData);
});
});
});
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