It isn't possible to set up a trigger based on text strings from a text ticket field. However, you can use other types of fields in your business rules, such as drop-downs or multi-select. For more information, see the article Using custom ticket fields in business rules and views.
Alternatively, you can also create a trigger based on text present in the subject field of your tickets. For more information, see the article: How do I create a trigger based on a ticket subject?