If you want to use a dynamic value in your event, use the "Value from page" building block in the Event data menu.

How to use "Value from page"?

Example: You want to identify all the visitors who search for "shoes" on your website's internal search.

1. Use the trigger "In page interaction", use the widget to select the search-button as your trigger, and choose to "click" as your interaction.

2. Use the condition "Value contains", and type in "shoes" as your value.

3. Use the event data "Value from page", and using the widget again, select the search field to define this as the element where the value is entered.

Now you have identified the segment of visitors who type "shoes" in your search field.

Further reading:

Creating your first events - 20 Minutes
How to install the Onsite widget?