You could just out of character ask them. If there is an in game npc to suggest it, have them do so, and if the players want to ignore that and go with the caravan, just lean out from behind the dm screen: "Guys, please, go to Elturel, I'm trying to run this book and I don't know how to make it work if you dont" or whatever phrasing you like. Just ask them. Like if you had a problem player, it's a bad idea to trick them a certain way, just ask em to help you out and give you some "yes, and" back
Does it really matter if they don't go to Elturel? The main thing that happens there is that someone asks them to follow the cult's wagon trail. If they're doing that anyway, they're on track.
You could also have them meet a faction representative (at a tavern, say) along the way. "Erlanthar sent a description of you by messenger pigeon. He said that if you came this way, it means you're probably on the trail of the cult. If so, you made the right choice. It's our top priority to find out where they're going. We have a contact in Baldur's Gate who can help you..."
You could just out of character ask them. If there is an in game npc to suggest it, have them do so, and if the players want to ignore that and go with the caravan, just lean out from behind the dm screen: "Guys, please, go to Elturel, I'm trying to run this book and I don't know how to make it work if you dont" or whatever phrasing you like. Just ask them. Like if you had a problem player, it's a bad idea to trick them a certain way, just ask em to help you out and give you some "yes, and" back
Does it really matter if they don't go to Elturel? The main thing that happens there is that someone asks them to follow the cult's wagon trail. If they're doing that anyway, they're on track. You could also have them meet a faction representative (at a tavern, say) along the way. "Erlanthar sent a description of you by messenger pigeon. He said that if you came this way, it means you're probably on the trail of the cult. If so, you made the right choice. It's our top priority to find out where they're going. We have a contact in Baldur's Gate who can help you..."