Arguments from Power Apps to flows are always of type strings, so you don't need the DateTimeValue (which converts from strings to date/time). You need the Text function, which allows you to format a date/time value in a specific way, something like the expression below:
Προσθ?κηστοημερολ?γιο.Run(
Text(DateValue1.SelectedDate, "yyyy/mm/dd") & " " &
Text(
Time(HourValue1.SelectedText.Value, MinuteValue1.SelectedText.Value, 0),
"hh:mm:ss"))
If the types of [HourValue1 | MinuteValue1].SelectedText.Value is not a number (i.e., they are strings), then you may be able to concatenate them directly instead of using the Text and Time functions:
Προσθ?κηστοημερολ?γιο.Run(
Text(DateValue1.SelectedDate, "yyyy/mm/dd") & " " &
HourValue1.SelectedText.Value & ":" & MinuteValue1.SelectedText.Value & ":00")
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