Nevermind - just found the solution. In order to set the dropdown values for the “From” dropdown, you need to use the “SetValue” method, use “activeEmailConfig” as a field ID, and pass an array of email mailbox IDs (as strings) as the “value” parameter. Example:
client.interface.trigger("setValue", {id: "activeEmailConfig", value: ["5000047958", "5000093390"]})
This field ID doesn’t seem to be listed in the Interface API docs for the “Ticket Details” page - I found it in docs for the Data API under the “email_config” method. Would be great to see it listed in the Interface API to avoid further confusion.