"보고 싶당" is an informal expression of "보고 싶다". In the Korean language, verbs end in "다" and people say "당" instead of "다" when they want to be cute funny. In other words, you can consider "보고 싶당" is a cute funny variation of "보고 싶다".

보고싶당 is funny pronunciation of 보고싶다.