A forum is basically a type of place, and was originally an area of town. So "in" is appropriate. "On" would only make sense if it was the sort of spot that you couldn't be inside, but could put things on top, on the surface. Most fora (the plural of forum — whether websites, physical meetinghouses, events that happen to be held in various locations, or whatever else) can definitely be entered, and likewise the idea of putting something on top of most of them in order to discuss it is rather strange.
This is even more true if the formal name of the event includes "Forum" and it's commonly referred to that way. In this case, you can also eliminate the definite article. So you might say, if it's clear enough,
"I want to raise a suggestion in Forum."
This might be the case if, for example, the event was called a General Meeting. That's both the type of event, and a good name to use to refer to it. However, if you're not sure whether this is going to be clearly understood as referring to the particular event, it's best not to use this. It's always correct to use the definite article and refer to the type of event.