Form Variables are not supported in Nintex Forms for Office 365 as of now. It will be coming in soon as we bridge our gap with parity between the two products. Stay tuned for announcements when its available. For now you can go with a calculated field and if you have any specific requirements, reach out to us in the community.
A workaround that I used today: add a Calculated value action and to avoid display value in you form, add a CSS class with the property 'visibility' (visibility: hidden) or put the action under another action ("Send to back" option in ribbon).