drag letter in empty box and check the string

hi, i need help i’m making a game like 4 pics one word

but just with one pic, i have empty box and below i have random letters, i can drag these letters but now i don’t

know how to put these letters in the empty box and to check the final word when it’s full

can someone help me, i can make transition to put letters in the empty square but i don’t know how to check