Do you have a particular need for Scriptable to do this? If not, asking Siri to remind you to run a particular Shortcuts custom shortcut might be easier, or using Launch Center Pro’s (LCP) scheduling.
If you do have a specific need to do it in Scriptable (e.g. to pass some data into Shortcuts), then you would need to have Scriptable open a URL as the action. The URL would need to match to the Shortcuts URL scheme and direct to run your custom shortcut. But if all it is doing is opening one web site, you could just open that URL from Scriptable instead.
If it is just a single web site URL, then LCP could do that just with its native features, but Shortcuts (+ Siri) and Scriptable have the advantage of being free … whereas you need to pay for extra LCP features.