example.ts

const agent = new BskyAgent({ service: 'https://bsky.social' })
await agent.login({ 'identifier': IDENTIFIER, 'password': PASSWORD })

const mentionLikeText = '@mention'

const text = `${mentionLikeText} This mention is URL link. I think this is really bad...`
const byteStart = 0
const byteEnd = (new TextEncoder().encode(mentionLikeText)).length

const richText = new RichText({ text })

if (!richText.facets) {
  richText.facets = []
}

richText.facets?.push({
  index: { byteStart: byteStart, byteEnd: byteEnd },
  features: [
    { $type: "app.bsky.richtext.facet#link", uri: 'https://example.com/some-bad-url' }
  ],
})

const facets = richText.facets

const postObj = {
  $type: 'app.bsky.feed.post',
  text,
  facets,
  langs: ['ja', 'en'],
}

await agent.post(postObj)