Sanford is right
form.addHiddenFields({ "Marketing_Detail__c":"fact sheet nuix collector suite"});
is a better option
Just make sure to remove he original Marketing_Detail__c field.
The method will append a duplicate field with a new value but the original field with the old value will still remain.
If this form is used elsewhere you may just want to set values using jquery
<script>
MktoForms2.loadForm("//app-sjf.marketo.com", "957-JZE-399", 2128,
function(form){
$('input[name="Marketing_Detail__c"]').val("fact sheet nuix collector suite");
form.onSuccess(function(values, followUpUrl){
location.href = "/download/fact sheet nuix collector suite";
return false;
});
});