Q: htaccess url parameter rewrite for specific page as subdomain I want to change my specific page URL as subdomain by .htaccess from
example.com/page.php to page.example.com.

A: We need to make some assumptions, since you've not included this information in the question:

*

*You are requesting (ie. linking to) page.example.com and you wish to serve /page.php.

*Both the subdomain page.example.com and domain apex example.com point to the same area of the filesystem, so both hostnames serve the same content.

In which case it is a relatively straight forward internal rewrite from page.example.com to page.example.com/page.php (based on assumption #2 above).
In your .htaccess file, you can achieve this using mod_rewrite:
RewriteEngine On

RewriteCond %{HTTP_HOST} ^(page)\.example\.com
RewriteRule ^$ %1.php [L]

The %1 backreference simply holds the value page from the preceding CondPattern.
This only matches requests for https://page.example.com/ and not https://page.example.com/<something>
