If you mean this one:
Robert G. Schaffrath - Professional Profile[
^] you can either find one of his messages:
Latest Messages[
^] and reply to it, or use the "Email" widget under the message body to send him a message.
Or, you can use his "@ name" to send him a message by entering it into a message - it's at the top right of his profile page - just as yours will send you one: @User-13742664
However, he doesn't appear to have logged in since 2014, so don't be too surprised if he doesn't reply, or if his email is not up to date.