Currently, the way to create hidden fields in the Metadata Platform is via the UTM Parameters in the Library. For example, you may want to tag leads coming into the platform with a specific SFDC Campaign ID.
First, navigate here
From here you will be able to configure your default UTM's which leads are tagged with as they flow into the platform.
Click Edit -> Choose the option for "Add Custom UTM"
Label your Parameter any custom value you would like, for example, "SFDC ID" and select "Custom" under the value column. Enter any value in the Facebook/LinkedIn channel.
Now create a draft Campaign and go to the settings tab and select UTM Parameters -> Edit.
From here update the value to the custom value you wish all Leads to be tagged with as they flow into the platform.
Now go to Integrations and view mapping for your current MA tool connected and configure the newly added custom UTM to the destination field in your MA tool.
That is all!
If you have any further questions about this feel free to reach out to firstname.lastname@example.org