PostgreSQL procedure to insert a sequence of random numbers in a column

I need to insert k random numbers (in the range 1:n) concatenated by | into a column of a table 'X', where n is the number of rows in table 'Y', in a PostgreSQL procedure.

To find the number of rows in the table 'Y'

select count(*) into n from Y

This will generate k random numbers in the range 1:n

SELECT num FROM GENERATE_SERIES (1, n) AS s(num) ORDER BY RANDOM() LIMIT k;

How do I concatenate the k integers with | and insert them into 'X'?