Jan Hendrik Koegler was just a very young carpenter during the Nazi occupation of the Netherlands, but in some way, he must have compromised himself, or he wouldn't have been temporarily barred from his profession after liberation in 1945. Whether or not this ban prompted his decision to become a singer, I don't know; in any case he started singing in musical revues as Jan Handerson in 1950.
From 1955, he was a much-beloved member of the Hoofdstad Operette, a (Jewish-owned) troupe that toured the entire country with (primarily Viennese) operettas. In the Netherlands, he (and not Johannes Heesters) was the prince of operetta. From 1964 to 1973, he sang at the Stadttheater Hildesheim, then he returned to the Hoofdstad Operette, and sang there until 1988 (comprimario parts in his last years). His most important role was Danilo, he sang it almost 1000 times.
In opera, he appeared but rarely, and not in standard repertory: the title role in Robert Kurka's The good soldier Schweik for the Dutch Opera Foundation in 1975, the Ballad Singer in Carlisle Floyd's Of mice and men at the Holland Festival in 1976.