To add in a hidden field, you'll need two things: the "Embedded Content" component, and a piece of code.
Add the embedded content into your step. It will come up as a blank square in your step:
Change the code in the code snippet to the below:
<input name="hidden_field_name" value="my_naughty_secret" style="visibility:hidden;height:1px;min-height:1px;" />
Feel free to change the input name and value to suit your needs.
Once the code has been added in, the blank square is now smaller and will not take up too much space in your flow:
For customers going through your flow, the hidden field won't be visible. However, it will now appear the developer tools and data will be sent through the API when information is submitted:
Congratulations! Enjoy some hidden parameters going through your flow!