Example: Set a watcher at ticket creation depending on custom field's value
You can do it adding the following two post-function to transition "Create issue":
Set a field as a function of other fields: to set auxiliary field "Ephemeral string 1" with the name of a user depending on the value of field "Customer".
Add or remove watchers: to add the user stored in "Ephemeral string 1" as issue watcher. Look at the screenshots:
Once configured, the transition will look like this: