The Twitter user paisley_trees, who is a history reseacher at Yale, has put our attention to this letter, which was sent by Süleyman the Magnificent to Dom Joao III of Portugal, who are both leaders in Civ6.
This letter actually concerns a trade agreement between both countries. Both Ottomans and Portugese had some confrontations regarding the domination of the Indian Ocean. After multiple failed confrontations from both sides, Süleyman the Magnificent wanted to make peace, and suggested a trade: They wanted 250 000 kg of pepper in exchange for 5-15 years of peace and 3 800 tons of wheat.
The Portugese made a counter offer, 125 000 kg of pepper in exchange for at least 15 years of peace, 7 600 tons of wheat, free passage for trading, and a demand to the Ottomans not to construct galleys suitable for the Indian Ocean.
The relevant letter here is Süleyman The Magnificent addressing Joao’s last request, rejecting it, and asking again for the first terms.
The whole letter is sadly not freely available, but most of it (without further discussion in the research paper) can be seen for free on the first page, and it is possible to download it after a free registration.
You can find this letter here: https://www.jstor.org/stable/41104901