Rive / Set Text actions replace the value of Rive text within a Rive animation. To read more, check out Rive's text support page.

For the Set Rive Text action to work, you need to previously expose the Text Run name in Rive Editor. To do that, select the Text Run, right-click, and choose “Export Name”. In Play, you'll use the Text Run name (not the TextBox object name) without brackets.

Select the Rive element that the action will affect.

Select the Rive file's text, which is created in Rive. You need to type the text name in exactly as it appears in the Rive file.

Set the new text value for the selected Rive text.