What word will people comment?

The word you ask for in your caption. One short word beats a sentence.

What should we send them?

This is the DM they get. Short and friendly wins.

We stick the link on the end of your message.

Where should this work?

Most people just leave it on every post.

Ask them to follow you first?

They get a Follow us button and an I've followed button. When they tap it we ask Instagram whether they really followed, and only then send the link. Anyone already following skips the step entirely.