The hidden field, when filled out may either update existing data in your contact record, or fill out a certain field set up with it.
You see a text message with a url in it, you click it and the link goes to a webform, then links it to the contact record.
In your webform with the Non-visible HTML element. Paste the code below to the HTML element:
Do not forget to add in a Hidden Email Field, and your Contact Email merge field ~Contact.Email~
Note: You may also use a Country Hidden Field.
In your http post snippet, add the webform link generated from your click tags set up.
View this link to see how to set up a click tag.
Once everything is set up correctly, and your customer fills in the webform it should update your contact record’s information, or fill in needed data.