How do I define the field Value of a Dropdown action?

Use what's in the attribute value in the HTML tag of your dropdown.

For example, there's a dropdown with the following values:

<option value="DOG">Dog</option>
<option value="CAT">Cat</option>
<option value="HAMSTER">Hamster</option>

To select Dog from the dropdown in your user flow, enter Dog in the field Value.