There is Probably a better way to do this, but I do it this way. Since it's a manual screen and not a form, forms have built in require options in the properties for fields. But on a manual check. I Do two things.
1. Add logic to my save button DisplayMode. I do a IF(isBlank(Textbox1.Text) || isBlank(Textbox2.Text),DisplayMode.Disabled,DisplayMode.Edit) This will make the button not active or clickable until the fields are field out.
2. Create labels near the field, and put in the *Requires a Value in red text, and make it's visable based on having a value using isBlank(Textbox1.Text)
You can access the Entity and mark specific fields as required. If a user attempts to submit the form they will be told that missing data is required. Using this setting should alwo include a visible * to indicate the field is required.