where is the field to look for a forum participant
There are thousands of them (if not millions!!) in there so you need to know something about the person concerned.
You can type their name in the Ask a question box and you will get a list of the posts they have participated in.
Some of these could be helpful:
Along with the info, that others have offered, if you have a thread, in which the desired person has contributed, just click on their Profile to the upper-left of the thread, and choose "Send Personal Message" (PM).