This snippet will let the bot react with custom emoji to every message sent in the specified channel. You will be able to specify the channel in the endpoint trigger.
const lib = require('lib')({token: process.env.STDLIB_SECRET_TOKEN});
let result = await lib.discord.guilds['@0.2.4'].emojis.list({
guild_id: `${context.params.event.guild_id}`,
});
let emoji = result[Math.floor(Math.random() * result.length)];
let emojiBuilder = `<:${emoji.name}:${emoji.id}>`;
let lastMessage = await lib.discord.channels['@0.3.2'].messages.list({
channel_id: `${context.params.event.channel_id}`,
limit: 1,
});
await lib.discord.channels['@0.3.2'].messages.reactions.create({
emoji: `${emojiBuilder}`,
message_id: `${context.params.event.id}`,
channel_id: `${context.params.event.channel_id}`,
});