Is it permissible to perform tayammum instead of ghusl for major ritual impurity (janabah) for someone who intentionally became ritually impure at their workplace, and the time for prayer has arrived but their workday has not ended?

It is obligatory for you to perform ghusl at work if the prayer time has arrived and you are in a state of janabah, and it is possible for you to perform ghusl. If ghusl is not possible due to the lack of a suitable place, and you can perform ghusl at your home before the prayer time expires, then delaying ghusl becomes obligatory. If you fear the expiry of the prayer time, then you must perform tayammum. There is no difference in the ruling of ghusl whether its cause was unlawful, such as masturbation, or lawful, while emphasizing the unlawfulness of masturbation.