This feature has been a part of HyperCard for some time, but is not documented.
Here's one way to take advantage of it:
You can write a handler that builds a find command in the message box, then
"types" return to execute the command. For example, here's a handler that
finds the next occurrence of whatever text has just been copied into the
message box:
on mouseUp
put "find whole"&"e&message"e into message
repeat 2
-- The first find will be the text you just copied into the message box;
-- the second find will be the next one, if any.
type return
end repeat
end mouseUp
Each time you then press the return key (without changing the content of the
message box), you'll go to the next occurrence of the same search string.