It should be made in a similar way as §83(b) and "no later than 30 days after the first date the rights of the employee in such stock are transferable or are not subject to a substantial risk of forfeiture, whichever occurs earlier", according to §83(i)(4)(A) and the Explanatory Statement of Conference Report, provided the employer is able to provide the notice required.

As with §83(b), there are potential downsides to making the election.  I would ensure the client fully understands the implications and have that documented before making the election.