The connector template and JS file need to be in the same directory and have the same name. So
/assets/javascripts/discourse/connectors/topic-navigation/loner-watch-topic-form.hbs
<form {{ action 'lonerWatch' content on='submit' }}>
<p>Enter your email address to watch this topic for updates</p>
<input name="email" placeholder="Your email"/>
<button>Watch</button>
</form>
and
/assets/javascripts/discourse/connectors/topic-navigation/loner-watch-topic-form.js
export default {
actions: {
lonerWatch(content) {
console.log(content);
},
},
};
Your action should then work. You can read a bit more here if you’re interested.