You don't. The whole point of working on a contract is that you trade a (substantially) higher bill rate for all the standard benefits of working as an employe

You don't. The whole point of working on a contract is that you trade a (substantially) higher bill rate for all the standard benefits of working as an employee. So no healthcare, no company provided machine on your desk, no paid vacation or sick leave, no retirement contribution, no invite to the xmas party, and no stock options or equity. Negotiate your contract with that in mind, and charge accordingly. When and if you decide to do the "to hire" part of the arrangement, negotiate that bit knowing that equity and free cake at Stan's birthday party are now back on the table. (Favor the latter, as it's likely worth more).