Once upon a time, there lived a celebrated Rishi called Rishi Vibhāndaka. He had a radiant son called Rishyashringa. Shringa lived, along with his father, segrated from the outside world. At the same time, there was a king called Romapāda, who ruled Angadesh. Ascribable to the king’s sins, the kingdom faced a terrible drought, which brought great misery to his people. At the advice of various learned Brahmins, the king decided to bring Rishi Shringa to his kingdom, for only then shall the drought end. But there was an impediment, Rishi Shringa was an ascetic. Bringing him to the kingdom was not going to be easy. Hence, Rompāda sends various maidens with splendid beauty to lure Rishi Shringa into the kingdom. The ladies of the night, after some efforts, managed to bring Rishi Shringa into the kingdom. As was predicted, the moment the sage set his foot into the kingdom, the rain lashed the dessicated grounds of Angadesh, bringing succour to the scorched land. The king duly welcomed Rishi Shringa and took him inside his palace. He got his daughter Shāntā married to the Rishi with due ceremonies. Hence, the king shall be blessed with a son-in-law like Rishi Shringa.
In the future, there shall be a devout king called Dashrath, who would rule Ayodhyā. He would befriend king Rompāda and then, with the help and blessings of Rishi Shringa, he shall beget several mighty sons to take forward his lineage.
Hence, as per the prophecy, Sumantra adviced King Dashrath to go to Angadesh and get Rishi Shringa perform a Putrakāmesthi yajna for him.
Therefore, king Dashrath, along with his family went to Angadesh, from where he brought Rishi Shringa and his Putrakāmesthi yajna was fructified with the birth of Shri Rāma and his brothers.
Source: Sarg 9–11, Bāla Kānd, Vālmiki Rāmāyana