How can I add a Pathauto pattern for a content type depending on a reference field?
Right beneath the pattern input you have a link that opens a popup to let you choose tokens to be put in the pattern. For field values it normally goes along the line of [node:field_foobar:value]. Depending on the field value you may also consider to either provide your own custom token or to alter the current token with an alter hook to replace the value with an URL-safe value (like to prevent the URL contains an Emoji for example).