SELECT STUFF('AD675498IJU76', 5, 0, '-')
SELECT STUFF(STUFF('AD675498IJU76', 5, 0, '-'), 8, 0, '-')
SELECT STUFF(STUFF('AD675498IJU76', 5, 0, '-'), 9, 0, '-')

DECLARE @hyphen       VARCHAR(100) = '2,10',
        @inputstring  VARCHAR(max) = 'ABCDEFGHIJKLMNOPQRST',
        @hyphen_count INT,
        @i            INT=1,
        @key          INT;

SET @hyphen_count = LEN(@hyphen) - ( LEN(REPLACE(@hyphen, ',', '')) ) + 1

WHILE @i <= @hyphen_count
  BEGIN
      IF ( CHARINDEX(',', @hyphen, 1) ) <> 0
        SET @key = SUBSTRING(@hyphen, 1, CHARINDEX(',', @hyphen, 1) - 1)
      ELSE
        SET @key = @hyphen

      SET @inputstring = STUFF(@inputstring, @key + @i, 1, '-')
      SET @hyphen= SUBSTRING(@hyphen, @key + 1, LEN(@hyphen))
      SET @i=@i + 1
  END

SELECT @inputstring