A field key is the unique, machine-readable name of a custom field. It looks like contact.loan_amount or opportunity.close_reason and is generated from the field's name when you create it. The key is what merge tags, workflow conditions, inbound webhooks and integrations use to find the field. Separate from the key, every custom field also has an ID, a random string the API uses in customFields arrays.
Two fields with the same display name in different sub-accounts can have different keys and always have different IDs. That's why an integration built for one client can fail when pointed at another.
In practice
To find a key natively, open the sub-account's Settings, go to Custom Fields, find the field and look at its details, or open the merge-field picker in an editor (as of October 2026). The field ID usually takes the API or the browser's developer tools.
Keys matter most when you wire GoHighLevel to something else: a Zapier step, a webhook from a form tool, or code that writes to contacts through the API. Renaming a field later doesn't reliably rename its key, so the label you see and the key the integration uses can stop matching. Check the key, not the label, before you build.
Where it shows up in RGS Tools
Field Finder's field browser lists every field's key, ID and merge tag, searchable and one click to copy, including fields on custom objects. Sub-account compare shows fields that exist in one sub-account but not the other, and keys whose type changed. CSV export gives you the full key list as a spreadsheet.