For me this question did not made any sense.
the sample code is just concatenating another variable to it, you can name it @salt to @sugar... still the sample code will not make sense to me.
And in your question, you say as SALT parameter, HASHBYTES does not has any salt parameter, you are just concatenating a variable (declares as salt) - which does not makes as parameter to it.
if you just use this, it gives different results
select hashbytes ('SHA1', 'FIRST')
select hashbytes ('SHA1', 'FIRST' + ' SECOND')
in both cases INPUT value is different, so its obvious the HASH return string will be different. (its a known thing)
My only concern is - question and it's answer does not really suites. I dont think SALT is tech word here in SQL, so it does not paints proper picture.
The first and the hardest SQL statement I have wrote- "select * from customers" - and I was happy and felt smart.