Send hidden text field

Hi,

I’m trying to do something i thought would be really simple, but i’m struggling with.

I’m using Contact Form 7 to create the forms and MC4WP to send the data to mailchimp, integrating both plugins.

I’ve created a field in my mailchimp list to identify what form is the info coming from in order to start an automatic email marketing process. It is a text field, named MMERGE7. The list is refreshed in MC4WP plugin.

Now i want to send a hidden text to that mailchimp field, but i’m not being able. I’ve been using the Contact Form shortcodes and the html i found in other topics here. For example:

<input type="hidden" name="mc4wp-MMERGE7" value="OK"/>

or

[hidden mc4wp-MMERGE7 default: "OK"]

and 

[hidden mc4wp-MMERGE7 default:text "OK"]

I cant make any of those work.

Could you help me with this?

Thanks a lot