Guru mantra is a mantra given to devotees by Acharya Maharajshree. One should take this mantra when they have a steadfast belief in Bhagwan. When taking your Guru mantra you must vow to follow 5 Vartman. Guru mantra is essential, without it salvation is very difficult to achieve.
Once the mantra is received it must be recited everyday in your pooja. This is done like a normal mala the only difference is that every bead you take instead of reciting Swaminarayan you recite the mantra.
The mantra used is from the Vedas
Guru mantra was given to Ramanand Swami by Ramanujacharya in his dream at Shreerangkshetra. He knew this dream was true as in the dream Ramanujacharaya had marked swamijis body with gopichandan. When swamiji woke up the gopichandan was present on his body.
Guru mantra was given to Dharmadev and Bhaktimata in Prayagraj(Trivedi Sangam) by Ramanand swami.
Guru mantra was given to Bhagwan by Dharmadev in Ayodhya. The Janoi was received at this time
When we take Guru mantra it is like we are getting married to Bhagwan and we must make sure that we keep that marriage going by reciting Guru mantra everyday.