Use htmx.onLoad()

Simple example:

<script src='https://unpkg.com/htmx.org@1.9.6'></script>

<p class='mt-5 mb-0' id='last-updated'>Last updated ...</p>
<button
  id='fetch-last-updated'
  hx-post='/partials/last-updated/'
  hx-trigger='click'
  hx-target='#last-updated'
  hx-swap='innerHTML'>
</button>

<script>
  htmx.onLoad(function (el) {
    const contentElement = document.getElementById('fetch-last-updated')
    htmx.trigger(contentElement, 'click', {})
  })
</script>