On March 3, 2019, in the first game of the season against Seoul E-Land, Um made his professional debut, coming in for the injured Kim Jeong-hwan at the 39th minute and being involved in a 2–0 win for his side.

Manager Park Jin-sub soon established him as a frequent starter of his side, as Um usually formed an attacking partnership with left winger Willyan and center-forward Felipe Silva.

On August 1, he scored his first brace in a 3–1 away win against Incheon United, and another one came on September 12, in a 3–3 draw against Jeonbuk Hyundai Motors: in this last occasion, his former goal was a volley lob over the head of goalkeeper Song Bum-keun, while the latter one was a shot in the bottom corner of the net after an assist by Lim Min-hyeok.

In 2018, he took part both in the Toulon Tournament, where the South Korean selection was eliminated in the group stage, and the AFC Under-19 Championship, where he helped his side reach the final by scoring a goal against Qatar in the semi-final.

Featured as a starter against China, Uzbekistan (in the group stage) and Australia (in the semi-final), he often found himself alternating with Lee Dong-jun on the right wing and eventually contributed to the Taegeuk Warriors' first victory of the competition, as they beat Saudi Arabia after the extra-time in the final and qualified for the Olympics.