This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 暤 is uncertain. It visually combines 日 (sun) with 皋, but the original semantic and phonetic roles are unclear.
The sun 日 beside 皋 suggests brightness, but the kanji's meaning is obscure. Think of it as a rare character for a bright, clear day.
For コウ, imagine the sun 日 shining brightly over a calm 皋 (こう) marshland: こう -> コウ.