Obtaining the Voivode's Letter of Safe Conduct in Kingdom Come: Deliverance 2's Miri Fajta questline requires navigating several steps. This guide outlines the process.
To acquire the letter, advance the Miri Fajta quest at the Nomads' Camp until the Voivode reveals Marika possesses it. Return to Marika in Apollonia. Assist her in repelling wolves.
During your conversation with Marika, select these dialogue options: "He cares about you," followed by "I swear." She'll then disclose the letter's hiding place: a tree near a nearby river. The location is illustrated in the screenshots below.

Retrieve the letter from the tree and return to the Nomads' Camp. However, before speaking with the Voivode, consult Tibor. He'll propose a wager for both the letter and an amulet.
This wager involves two challenges: a horse race and a fistfight against Tibor. To improve your chances, consider acquiring the "Good Old Pebbles" perk beforehand for the race, and participate in unarmed melee tournaments to enhance your fighting skills. While Tibor isn't exceptionally difficult, he's a more skilled opponent than average.
This completes the acquisition of the Voivode's Letter of Safe Conduct. For additional Kingdom Come: Deliverance 2 strategies and guides, including obtaining the Royal Treasury Key and romancing Katherine, check out The Escapist.