Ninja Forms field does not transmit the value

`Hello all,
I want to transfer variables from the URL in a Ninja form.
For this I use the GTM and create a cookie to store the URL parameters and use them as variables.
To fill the Ninja form field I use the following javascript command:
document.getElementById(“nf-field-12”).value = “{{FPC_utm_campaign}}”;
The URL is read, the parameter is identified, and the variable is written to the specified field, but when the form is submitted, the value is empty

Does anyone have an idea what the problem is, because I’m not getting anywhere unfortunately.
You are welcome to test it at
https://bsend.de/?utm_campaign=Brand_Campaign and then click on contact.

The UTM Campaign parameter should be displayed in VAR2 when you test it.

Thanks in advance
best regards
Benjamin

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