You can do something like this
find -L . -type f -name "*.txt" -exec bash -c 'echo "$0" "${0%/*}/PrependedTextHere${0##*/}"' {} \;
Where
bash -c '...'
executes the command$0
is the first argument passed in, in this case{}
-- the full filename${0%/*}
removes everything including and after the last/
in the filename${0##*/}
removes everything before and including the last/
in the filename
Replace the echo with a mv once you're satisfied it's working.