To pass values as a comma separated string, you need to concatenate them into one comma separated string and pass it.

To use it in you procedure, you can use "IN" clause with Dynamic-SQL, or use some string splitter functions (you can find good one on this forum) to split into table.

And as I said before, "IN (@Paremeter)" is exactly the same as "= @Paremeter", and it is irrelevant of what you have inside of @Paremeter.