Your employer is exempt from its own tax, not withholding tax of its employees. It has to withhold tax from employees salary essentially and pay to FBR on employee's behalf. Please check your payroll slip and you will know how much tax is deducted from your salary each month.
Also, request your employer to provide you Withholding tax certificate. It contains the tax deducted each month and the dates and other details of when the same was deposited in National Bank to the credit of FBR.