This is a vocation, not a job. This tends to be influenced by type of union. There are more men than women in senior positions. Research and writing skills are essential. A degree related to the union's area of work may be useful.
Some unions specify degree level qualification.
Try to take modules that bear some relation to trade union work. Relevant work experience or trade union involvement will improve your chances. A pre-entry postgraduate qualification is not needed Undertaking research related to union work may enhance prospects. Postgraduate qualification in industrial relations has some relevance. Work in a related area may help.