insert one dash where needed.\n—\nthe trade unions president was voted\ninto office by a resounding majority…

Brief Explanations:

To insert the dash, we need to identify the break in the sentence where a dash (em dash or en dash, here a single dash as per instruction) is appropriate to show a pause or interruption. The part "at least" starts a new thought related to the previous statement, so we place the dash after "majority".

Answer:

The trade union's president was voted into office by a resounding majority — at least that's the story they gave.