The easiest way of doing this is to prefix the field with a formatting tag: e.g.
<mystyle>text text text</mystyle>
If you set the field to be ‘Formatted’ in its Field Options, you should find that the character style named ‘mystyle’ will be applied to the text in the example above. If you have a field containing the name of the style to apply, you could enter:
in the field’s prefix and
in the field’s suffix.