"Link to" is the language that made this confusing, because phpBB does indeed let you link to a specific post. (But in context of the topic it was posted in.) "
Show only a single post" is what's actually trying to be achieved here, and for which phpBB doesn't appear to already have that solution.
I got excited because search.php actually had an internal "p=" post ID parameter it would recognize, and I thought maybe you could use a search.php URL to isolate that one post & show the entire post. But it doesn't appear to work that way, so no dice.
If the post you wanted to isolate
just happened to be the only post
by you in that topic, you could use search.php to show only posts authored by you in a specific topic ID. Such as for this topic we're posting in right now, although clearly you have multiple posts in this one:
search.php?author=php_mike&t=2547106&ch=-1
Here is a different one where there is only a single post by you:
search.php?author=php_mike&t=2182462&ch=-1
But that's probably not what you're really needing. Using search.php doesn't put you in viewtopic.php where you could immediately "Reply" or similar. But unfortunately there is no "viewpost.php", nor does viewtopic.php appear to have a "display just a single post" mode.