Use the symbol జ్ఞా (U+0C1C U+0C4D U+0C1E U+200C U+0C3E). It's a combination between the consonant ja (జ), a virama ( ్ ), the consonant nya (ఞ), a zero-width non-joiner, and the vowel aa ( ా).
More information: Manish Goregaokar’s blog