[Plugin: WordPress Form Manager] Custom Field into Database via hidden field

Sorry for the late reply. That is going to be a tough one unfortunately, first you would have to add a column to the data table for your form, and then hook into ‘fm_form_submission’ and update the most recent record to add your custom field.

If I understand what you want, you should just make an extra field for your form and give it a nickname. Then you have some options:
1. Go to ‘conditions’, and do ‘Hide elements if…’ then choose ‘Always’, and then the hidden item under ‘Applies to’. This will keep the element hidden, but you can still change the value using JavaScript
2. Use CSS to hide the element. The ‘li’ tag will have the id ‘fm-item-(nickname)’ if you give it a nickname, so you could set the display to ‘none’ within CSS and modify the value using javascript.
3. Make a custom display template (its in the FAQ) and put some logic to do whatever you need for that particular field. You can test the item’s nickname in the loop.