Ἐν ἀρχῇ ἦν ὁ Λόγος, καὶ ὁ Λόγος ἦν πρὸς τὸν Θεόν, καὶ Θεὸς ἦν ὁ Λόγος. (John 1:1)
We have a preverbal anarthrous predicate nominative in John 1:1c along with the expression πρὸς τὸν Θεόν in 1:1b: the Word is with the God. However, there is a different construction in John 13:3. There, we find ἀπὸ + the genitive noun Θεοῦ, which suggests definiteness despite being anarthrous. It is not uncommon to find definite anarthrous nouns that are made such by virtue of prepositions.
In the latter part of John 13:3, we encounter τὸν θεὸν (signifying definiteness) describing the one earlier referenced by the anarthrous Θεοῦ. So how we render a scriptural passage involves more than whether the article is omitted: other syntactical and contextual factors must also be considered, but NWT critics generally overlook these factors.