Yea,

You can autoclaved D-mannitol. Because, mannitol cannot desolve without heating in water. After autoclave you can add mannitol in media in warm condition as a desired concentration.

As Alam said.

you can add Mannitol to the media and then autoclave it.

Yes, you can add mannitol to the media and then autoclave it. I did the same with my anther culture media..

All the best

Thank you all for your answer. It's really helpful