While there are Visual Automations in your account in your main Automations navigation tab, there are also Automation Rules. These were our very first version of Automations before we made the Visual Automations.
Here's a short video about rules!
New as of February 2019
You can now set a custom field using a rule!
Under the Action option you'll now see the choice to Set a Custom Field.
Once you select that option you can choose which custom field you'd like to add to and what the content of the field should be.