What is the story of Indrajit (Meghanada) who defeated Indra and hence received his title.
Meghanatha got epithet "Indrajit" after defeating Indra through illusory power he acquired from Lord Shiva. Indrajit took Indra to Lanka as captive. Brahma confers the name "Indrajit" to Meghanatha and talks a deal to release Indra. This is described in detail in Chapter 29 and Chapter 30 of Uttara Khanda of Ramayana.
When Ravana was obstructed by Indra, Indrajit used illusory power he learned from Lord Shiva and saved his father. Indra mounted on elephant Iravata to search for Meghanatha (as Meghanatha wasn't visible to Indra) and Meghanatha stuck him with many arrows. When Indra was exhausted, he took Indra as captive.
Chapter 29 says
ततो रथं समास्थाय रावणिः क्रोधमूच्छितः।
तत्सैन्यमतिसंक्रुद्धः प्रविवेश सुदारुणम्।।२०।।
Thereupon ascending his car Ravana's son, became senseless by his wrath and beside himself with rage, entered the dreadful flank.
तां प्रविश्य महामायां प्राप्तां पशुपतेः पुरा।
प्रविवेश सुसंरब्धस्तत्सैन्यं समभिद्रवत्।॥२१ ।।
Having resorted to the illusory powers, conferred upon him yore by Pasupati he entered into the enemies' camp and belaboured them.
स सर्वा देवतास्त्यकत्वा शक्रमेवाभ्यधावत।
महेन्द्रश्च महातेजा नापश्यच्च सुतं रिपोः।।२२।।
Having left behind all other deities he pursued Indra and the highly effulgent Mahendra too espied his enemy’s son.
विमुक्तकवचस्तत्र बध्यमानोऽपि रावणिः।
त्रिदशैः सुमहावीर्यैर्न चकार च.किंचन ।।२३।।
And albeit assailed by the highly powerful deities, Ravanas son, divested of mail, entertained no fear.
स मातर्लि समायान्तं ताडयित्वा शरोत्तमैः।
महेन्द्र बाणवर्षण भूय एवाभ्यवाकिरत्।२४।।
Having overpowered the approaching charioteer Matall, with many excellent arrows he covered Mahendra with a downpour of shafts.
ततस्त्यक्त्वा रथं शक्रो विससर्ज च सारथिम्।
ऐरावतं समारुह्य मृगयामास रावणिम्।।२५।।
Thereat having left his car and charioteer Indra mounted his elephant Airavata and ran about in search of Ravana's son.
स तत्र मायाबलवानदृश्योऽथान्तरिक्षग:।
इन्द्रं मायापरिक्षिप्तं कृत्वा स प्राद्रवच्छरैः।।२६।।
Being invisible in the welkin by virtue of his illusory powers and having brought Indra under the influence thereof he struck him with hundreds of arrows.
स तं यदा परिश्रान्तमिन्द्रं जज्ञेऽथ रावणिः।
तदैनं मायया बद्ध्वा स्वसैन्यमभितोऽनयत्।।२७।।
When Ravana's son came to know that Indra was exhausted he, having bound him up by virtue of illusion, proceeded towards his army.
तं तु दृष्ट्वा बलात्तेन नीयमानं महारणात्।
महेन्द्रममराः सर्वे किं नु स्यादित्यचिन्तयन्।।२८।।
Having seen Mahendra carried away by force from the battle field the deities thought “What is this?’
दृश्यते न स मायावी शक्रजित्समिर्तिजयः।
विद्यावानपि येनेद्रो माययापहतो बलात्।२९।।
That conqueror of Sakra and subduer of enemies, conversant with illusory powers, was not visible, by whom, Indra, although master of many illusions, was carried away by force.
Meghanatha said the following to Ravana after defeating Indra.
अयं हि सुरसैन्यस्य त्रैलोक्यस्य च यः प्रभुः।
स गृहीतो देवबलाद्भग्नदर्पाः सुराः कृताः।।३४।।
By virtue of my illusory powers I have made Mahendra captive the lord of the three worlds and of the celestial host and have crushed down the pride of the deities.
When all Devas went to Brahma, Brahma appeared to Meghanatha and made a deal to exchange Indra, conferring title of "Indrajit" unto Meghanatha.
Chapter 30 says
Brahma said to Ravana
जितं हि भवता सर्वं त्रैलोकयं स्वेन तेजसा।
कृता प्रतिज्ञा सफला प्रीतोऽस्मि ससुतस्य ते।।४।।
You have, by your own prowess, conquered the three. worlds- your promise has borne fruits- I have been pleased with you- both the father and son.
अयं च पुत्रोऽतिबलस्तव रावण वीर्यवान्।
जगतीन्द्रजिदित्येव परिख्यातो भविष्यति।।५।।
O Ravana, this your son is highly powerful and gifted with great strength and he shall be celebrated in the world under the appellation of "Indrajit' or the conqueror of Indra. irrepressible, by those help, O King, the celestials have been brought under your subjection.