Is it permissible to benefit from the healthcare services provided by the health insurance company with which the questioner's employer has contracted to provide health coverage for him and his family?

As long as the contract stipulates the employer's commitment to provide medical treatment for the employee and their family, the employee has the right to demand it and benefit from it if it is provided. If the employer referred the employee to an insurance company, this does not prohibit the employee from receiving their right that was referred. The sin would then be upon the employer, not the employee, unless the employee was given a choice between the employer paying for their treatment costs directly or referring them to insurance. In that case, the employee is not permitted to choose the insurance unless it is a cooperative, takaful, and permissible one.