Is it obligatory to confine oneself to performing only the obligatory prayers at work to ensure the permissibility of one's salary, and to perform the supererogatory prayers only at home, or is it permissible to perform the supererogatory prayers at work?

It is permissible for you to pray the regular Sunnah prayers; in fact, it is required, due to the reports of the Prophet’s (peace be upon him) consistent performance of them. Ahmad narrated from Ibn Umar, who said: "The prayer of the Messenger of Allah (peace be upon him) was that he would not omit two rak'ahs before Dhuhr, two rak'ahs after Maghrib, two rak'ahs after Ishāʾ, and two rak'ahs before Fajr." This is permissible as long as it does not lead to negligence in work. The jurists have stipulated that a private employee is allowed enough time from his work for his purification, the obligatory prayer, and the regular Sunnah prayers.