BasicUI and sitemaps have no support for a text field.
New in OH 3.3 is the Time is <item> trigger (see Rules | openHAB). This will trigger the rule based on the state of a DateTime Item.
However you are somewhat stuck if you are using BasicUI because there is no way to enter a DateTime. The best you can do is to define a separate Hours, Minutes, and Seconds Item with a slider. Changes to these Items trigger a rule where you’ll have to use the new values to build that DateTime Item’s state.
Search the forum for one of the many “alarm clock” examples to show how to do this.
If you use MainUI, all you need is to install DateTime Standalone Widget and DateTime List Item (or configure these yourself, there isn’t anything special or really all that custom about either of these) for DateTime entry on your MainUI widgets.
No.