Help | Why aren't URL's showing up?
When Slack sends us your message to Schedule, we can't see any links that were embedded
Unfortunately Slack does not support embedded links in its response to apps. So when you schedule a message, we have no way to know if you have included a link in your message.
If you need to schedule a Slack message with a link included, the best option is to included the full URL of the link instead of embedding it in your text. Ie:
/schedule here is the website - www.google.com in 5 hours
Do not embed a URL using this option if you would like your Url to be included in your scheduled message:
If you would like to include a URL in your scheduled messages, use the Advanced Scheduler.