This has come up before. I think the issue is that the data that’s posted in a Discourse webhook isn’t in the form that Discord is looking for. There are details about the issue in the first two replies to this topic: Has any one tried webhooks with Discord?.
You could point the webhook at Zapier, then use Zapier to parse the webhook into the form that Discord is looking for. That would cost at least $20/month though.
I don’t think it will help for your current case, but a good way to post from Discourse to Discord is outlined here: Set up Discord notifications with the discourse-chat-integration plugin.