"sarang hamnida" is a polite way of saying "I love you".
"sarang handago" means "that I love" and it's kind of quotation.
사랑한다고 말해 줘.
sarang handago malhae jwo.
-> Tell me that you love me.
A: 뭐라고? (mworago?) : What did you say?
B: 사랑한다고! (sarang handago) : (I said that) I love you!
Hope this helps!