What is the ruling on performing Umrah without official leave from work after a leave request has been denied?

A Muslim must fulfill the contracts he enters into, as Allah Almighty says: "O you who have believed, fulfill [all] contracts," and the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions, except for a condition that forbids what is lawful or permits what is unlawful." Therefore, if the employment contract stipulates that absence without leave is not permissible, then you are not permitted to do so.