T-SQL Convert multiple rows to comma separated String

The below query returns 17 unique reference numbers from 400+ rows

select 
Distinct(Ref_Num) 
from DAY_943 where Unique_Ref = '00138AMCR2018-04-21'

Select allOpen in new window

I need to output this as a single comma separated string.
By googling I see methods using CTE and XML Path, but not having much luck
Can anyone give me a hand with this Please.

I am using Mangement Studio 2014