Jains believe that the knowledge about Simandhar Swami (the living Tirthankar in Mahavideh Kshetra) and his role in helping Bharat Kshetra comes from the teachings of Mahavira, which were later revived and clarified by Jain Acharyas such as Kundakund (Kundakunda) and others in later centuries. In short:

• Who first knew: Mahavira is regarded as the source who disclosed that there are Tirthankaras in Mahavideh Kshetra (where Simandhar Swami resides) and that Bharat Kshetra could be helped by such beings in the future. This foundational disclosure is attributed to Mahavira and is considered part of the later “nav-tara” lineage of teachings that were preserved and expanded by later Jain scholars.
• In what way: The knowledge is presented as a cosmic, doctrinal fact about different worlds (Bharat Kshetra here and Mahavideh Kshetra elsewhere) and about the continuity of Tirthankaras and their compassionate activity for other realms. Later texts and teachers recounted and clarified how Simandhar Swami would assist Bharat through his presence and guidance, especially when no arihants are born in Bharat for long periods.
• When first known: The original disclosure traces to Mahavira in the traditional Jain narrative; the revival and explication of this idea, including specifics about Simandhar Swami and his role for Bharat, appear in later works such as the commentaries and biographical traditions that name Kundakunda and other later authors as revivers and expositors of this doctrine.

Note on traditions:

• Digambar and Shvetambar lineages sometimes differ in emphasis or phrasing about Mahavideh Kshetra and Simandhar Swami, but both acknowledge that a living Tirthankar resides in Mahavideh Kshetra and that his help is directed toward Bharat.

If you’d like, I can summarize a specific Jain scripture passage that mentions Simandhar Swami and his connection to Bharat, from a canonical or well-known commentary (with the exact wording in that tradition).