Populating WHERE clause in Dynamic query

In the following query I am having difficulty populating the WHERE clause.
How do I populate a_ActorID without hardcoding over a thousand a_ActorID's?

DECLARE @cols  AS NVARCHAR(MAX)
DECLARE @query AS NVARCHAR(MAX)

SET @cols =   (SELECT DISTINCT 
                      ',' 
                     + ' COUNT( case when ap.a_AssignmentsIdName='
                     + ''''
                     + a_AssignmentsIdName 
                     + ''''
                     + ' then ap.a_ActorID end ) as '
                     + QUOTENAME(a_AssignmentsIdName) 
                   FROM a_partsandincrements
                   WHERE a_ActorID IN ('143939', '128473')
                   FOR XML PATH(''), TYPE
                  ).value('.', 'NVARCHAR(MAX)') 

set @query = 
    + ' SELECT ap.a_ActorID , ap.a_ActorIdentifierName as Actor'
    +     @cols
    + ' FROM a_partsandincrements AS ap'
    + ' WHERE ap.a_ActorID IN (''143939'', ''128473'')'
    + ' GROUP BY ap.a_ActorID , ap.a_ActorIdentifierName'


execute(@query)
;

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