Hi @alhilo ,
1)You could check the control name on the left side:

On your picture, you should choose that control and find it here:

2)Do you want to :
if you select data in end date's datepicker earlier than data in start date's datepicker , you will get notification and the form could not be submitted?
If so, you should set like this:
set end date datepicker's OnChange:
If(startdatedatepickername.SelectedDate>=enddatedatepickername.SelectedDate,
Notify("please do not select date earlier than start date",NotificationType.Warning)
)
//please replace with your startdatedatepickername and enddatedatepickername
set SharePointIntegration's OnSave:
If(startdatedatepickername.SelectedDate<enddatedatepickername.SelectedDate,SubmitForm(SharePointForm1), Notify("select wrong date"))

Best regards,