Hidden Fields not pulling through

Query parameter names don’t include delimiters like ? and &! They’re the names alone, like utm_medium and utm_campaign. (This isn’t a Marketo thing, it’s true of all URLs.)

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When I've tested this with either of the parameter names it doesn't so any additional columns in the lists created from the form. I'm assuming that the additional fields I've added would automatically generate a new column on the list?

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You add the columns to the view yourself. Marketo won’t add custom columns for you — that would mean all views would contain hundreds of columns.

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And this is where I would check if the hidden fields are filling in properly. But I can't see any as of yet. 

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For  debugging form fills on the server side you first look at the details of the Filled Out Form activity in the Activity Log. That shows all the fields that were present on the form.

The columns in a Smart List view show the person’s current values. Those aren’t necessarily the same as the values submitted with their last form, because (for one example) Block Field Updates might have prevented the field from changing. The values on the form are always shown in the Filled Out Form details.

For debugging form values on the client side, you can use the Dev Tools inspector or other more robust methods.

My Dev team have added in some code but I wasn't sure if the name needed to match what I have as my Parameter name? So I'm not sure how best to advise them. 

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Please don‘t have your dev team add anything to your pages. They are very unlikely to understand the Marketo Forms 2.0 event model and will just add confusion. Simply filling hidden fields from query parameters does not require custom JS. Many advanced requirements do require custom JS, but not this one.