What does ようにする add here? Why not just say 掲げる?
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This ように is "like" rather than "in order to". In other words, it indicates this gesture of a referee is something that resembles 掲げる but not the most typical 掲げる action. 手を掲げる usually means raising one hand high over one's head, as shown in these image search results. The gesture described in your sentence should be slightly different from this.