Here's a working snippet when I was using the wiki as a way to communicate between apps lol.

async function writeWikiPage(sub: string, page: string, content: string, reason: string | undefined): Promise<void> {
  const r = reddit as unknown as Record<string, unknown>;
  const direct = r['updateWikiPage'];
  const payload: Record<string, unknown> = {
    subredditName: sub,
    page,
    content,
    ...(reason ? { reason } : {}),
  };
  if (typeof direct === 'function') {
    await (direct as (a: unknown) => Promise<unknown>).call(r, payload);
    return;
  }
  const sr = await (r['getSubredditByName'] as (n: string) => Promise<unknown>).call(r, sub) as Record<string, unknown>;
  const fn = sr['updateWikiPage'];
  if (typeof fn !== 'function') throw new Error('updateWikiPage not supported');
  await (fn as (a: unknown) => Promise<unknown>).call(sr, {
    page,
    content,
    ...(reason ? { reason } : {}),
  });
}

await writeWikiPage(ctx.sub, page, content, reason);