Is it permissible for me to market for other companies within my workplace without the knowledge of my company, and is the salary I receive for that permissible (halal)?

It is not permissible to engage in marketing for other companies while employed by your current company, except with the permission of the employer or the authorized manager. This is because such an employee is considered a "private hire" (أجير خاص) by the jurists, meaning someone who works for a single individual for a specific period. The jurists have stipulated that a private hire is not allowed to spend the contracted time on anything other than the company's interests, except with the permission of the responsible party. This is because their services during this period become due to their employer, as mentioned by Az-Zayla'i in "Tabyin al-Haqa'iq."