Further to solution 1 - unfortunately there is no out-of-the box format that will do that for you, you will also need
substring[
^]
e.g.
select SUBSTRING(CONVERT(varchar(30), GETDATE(), 121), 1, 16)
or
DECLARE @T AS TIME = '10:57:43'
select SUBSTRING(CONVERT(varchar(30), @T, 121),1,5)