Also support Discord.

app/Support/Notifications/UrlValidator.php

@@ -0,0 +1,39 @@
+<?php
+/*
+ * UrlValidator.php
+ * Copyright (c) 2023 james@firefly-iii.org
+ *
+ * This file is part of Firefly III (https://github.com/firefly-iii).
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+namespace FireflyIII\Support\Notifications;
+
+/**
+ * Class UrlValidator
+ */
+class UrlValidator
+{
+
+    /**
+     * @param string $url
+     *
+     * @return bool
+     */
+    public static function isValidWebhookURL(string $url): bool
+    {
+        return str_starts_with($url, 'https://hooks.slack.com/services/') || str_starts_with($url, 'https://discord.com/api/webhooks/');
+    }
+}