In Ephesians 6:20, Paul calls himself an ambassador in what?

Answer: Chains

Paul asks for prayer to proclaim the mystery of the gospel boldly, "for which I am an ambassador in chains" (KJV: in bonds).

“Pray also for me, that whenever I open my mouth, words may be given me so that I will boldly make known the mystery of the gospel, for which I am an ambassador in chains. Pray that I may proclaim it fearlessly, as I should.”— Ephesians 6:19-20 (BSB)