If message content mentions client user, bot will send prefix.

events/message.js

@@ -16,6 +16,16 @@ module.exports = {
                 prefix = prefixdb[0].prefix;
             };
             if (message.channel.type === "dm") return;
+            if (message.content.toLowerCase() == `<@!${client.user.id}>`) {
+                message.channel.send(`My prefix is ${prefix}`).catch(() => {
+                    return;
+                });
+            };
+            if (message.content.toLowerCase() == `<@${client.user.id}>`) {
+                message.channel.send(`My prefix is ${prefix}`).catch(() => {
+                    return;
+                });
+            };
             let args = message.content.slice(prefix.length).trim().split(' ');
             let cmd = args.shift().toLowerCase();
             let command;