How do I include a STAR in an ICAO flight plan?
ICAO Doc 4444 Appendix 3 has the clearest text (and an example):

Standard arrival route
The designator for the standard arrival route from the point of leaving the defined route to the point at which the approach procedure is initiated.
Note.— Standard arrival route need only be included where appropriate.

Let's break down the language:
&quot;The designator for the standard arrival route from the point of leaving the defined route to the point at which the approach procedure is initiated.&quot;
Which means:
(defined route – airway)
(last applicable point on airway)
(first point on STAR)
(STAR designator)
In the example given (below), both points are shared (FTM), similarly for the SID – emphasis mine:

N0460F290 LEK2B LEK UA6 FNE UA6 XMM/M078F330 UA6N PON UR10N CHW UA5 NTS DCT 4611N00412W DCT STG UA5 FTM FATIM1A


(Highlight mine.)