How can one perform the Sunnah prayer before Isha, if the prayer is established immediately after the adhan?

There is no regular Sunnah prayer before the Isha prayer. Rather, it is recommended to pray two rak'ahs before it, based on the Prophet, peace and blessings be upon him, who said: "Between every two calls [to prayer] there is a prayer." (Agreed upon). What is meant by "two calls" is the Adhan and the Iqamah. If the prayer is established, then the supererogatory prayer before it is missed, because an absolute supererogatory prayer is not made up.