Gurumantra, or Guru Mantra, as the name indicates is a letter, a word or group of words that a Guru suggests to a student or Shishya when he/she is accepted as a disciple. Usually, a Gurumantra depends on the character of the student, especially the Gunas - sattvic, rajasic and tamasic. A true Guru easily gauges which Guna is dominating the disciple.
Now, finding a true Guru is major issue. True guru never advertises; he/she is a like a wildflower and students or disciples are like bees and butterflies that search for the fragrance and reaches the flower even if it is hidden in the darkest spot in the jungle. True Guru is not a magician and he never performs miracles to attract disciples.
The sole aim of the Gurumantra is to control the senses and set the individual in the path of Brahman consciousness.
Those people who want to gain wealth and climb up the social ladder will never be initiated by a true guru.
Such disciples usually never find the true guru and instead land up at the feet of cunning individuals, who make a living by disguising as a Swami or Sanyasin. Such Gurus are like plastic flowers; it will look good but will not have the fragrance.
The Guru you accept will always be the true reflection of your mind. Fortune seekers will find such a Guru who is a more cunning fortune seeker than the disciple.