Hi patel,
Here are the steps.
1.Get your portal registration page url.

In my case, it's xxx.powerappsportals.com/Account/Login/Register?returnUrl=/ ("%2F" decode is "/")
2.Create a new web template, and add the following code in it.
{% if user %}
<script>
window.location.replace('');
</script>
{% else %}
<script>
window.location.replace('/Account/Login/Register?returnUrl=/');
</script>
{% endif %}

3.Go to Page Templates, find "Home" and change its web template to your new web tempalte.

4.Wait a second and refresh your portal, you will find that your portal home page(landing page) has changed to registration page.

Regards,
Leo