To add a custom field, on the Web Portal,
To create Contact Variables Settings cog > Contacts > Contact Custom Fields. These are variable fields relating to a contact for example birthday or any else you would like to capture about the contact. A client variable can be selected to be searchable in the contacts list.
To create Location Variables > Settings cog > Contacts > Location Custom Fields. This is specific information about the location itself. A location variable can not be selected to be searchable in the contacts list.
NOTE - a variable is only able to be pulled in through a form and visible on the contact card. If you are looking to store information on a location such as the Gate Access code, that would be stored as a location note which is easily visible on the App in the job.