Assigning text to a text field at runtime

Using the Text tool, create a text field on the Stage.
With the text field selected, in the Property inspector (Window > Properties), enter headline_txt in the Instance Name text box, directly below the Text Type pop-up menu on the left side of the inspector.
Instance names can consist only of letters, underscores (_), and dollar signs ($).
In the Timeline, select the first frame in Layer 1 and open the Actions panel (Window > Development Panels > Actions).

Type the following code in the Actions panel:

headline_txt.text = "Brazil wins World Cup";