The phrase "in a forum" is correct and can be used in written English.
For example, you could use it in a sentence such as: "I asked my question in a forum and received many helpful responses."
Mr. Fenty conceded in a forum last week that the episode "shouldn't have happened".
Explain that in keeping with her suggestion, they will participate in a forum on water pollution.
"But then she blossomed in a forum where she could explore and explode her poetic gifts".
In January, Mr. Obama answered questions from YouTube users in a forum sponsored by Google.
In a forum after the performance, the student actors confirmed as much.
Six weeks ago, I participated in a forum at Yale on the massacre in Charleston.
It is a magnificent folly in a forum of serious debate.
Recently, we participated in a forum (about the new health care law) where my progressive politics became obvious.
All parties — the offender, victim, facilitator and law enforcement — come together in a forum sometimes called a restorative-community conference.
WHERE did these women participate in a forum to celebrate the milestone of an all-female delegation?
