That’s probably not actually legal. The determination of whether you are an employee or an independent contractor should be made based on your duties and the relationship between you and the employer, and it is not really subject to negotiation unless you are negotiating the entire package—not just your wage but also how your work will be controlled and what your duties are.
https://www.irs.gov/businesses/small-businesses-self-employed/independent-contractor-self-employed-o...
For example, as an employee, you might be paid a certain amount per hour or per month regardless of how long it takes you to finish a particular task. As a contractor, you might be paid by the task regardless of how long it takes. If you can complete the task faster than the company expects, you should receive the same amount of pay and you should be able to use that free time to freelance for another employer.
Just considering employment taxes, you would need at least 15% more as a self-employed contractor than your base salary would be as an employee. However, you may want to include additional compensation to make up for the fact that you are not receiving paid vacation, or subsidized medical insurance, or an employer match to a retirement plan or pension. At my particular employer, the value of all my benefits is considered to be an additional 40% on the top of my gross salary. There is no right answer, it depends on what you can negotiate for yourself.
