IDENTITY is supported on a memory-optimized table, as long as the seed and increment are both 1 (which is the default). Identity columns with definition of IDENTITY(x, y) where x != 1 or y != 1 are not supported on memory-optimized tables.
To increase the IDENTITY seed, insert a new row with an explicit value for the identity column, using the session option
SET IDENTITY_INSERT table_name ON. With the insert of the row, the IDENTITY seed is changed to the explicitly inserted value, plus 1. For example, to increase the seed to 1000, insert a row with value 999 in the identity column. Generated identity values will then start at 1000.