Create table function SQL Server

I would like to create a function which runs a specific query that will extract any DimPatientID that is listed within the DimTestPatient table. Currently, I use the WHERE clause to exclude the test IDs from my query. I would like a function because I will use the WHERE clause on almost any query that I will run in the future.

SELECT 
    COUNT(*)   
FROM 
    dbo.DimPatient dp
LEFT JOIN
    dbo.FactTreatmentHistory th ON dp.DimPatientID = th.DimPatientID
WHERE
    dp.DimPatientID NOT IN (SELECT DimPatientID
                            FROM dbo.DimTestPatient)