If going through Shopifypreview.com website, there is nothing you can do.  The only work around is go to Edit Code, then hit the "Preview store" at the top-right, that way you can view the temp theme via live domain and it MAY work as expected.  

Just want to confirm if Shopify will set every page to robots noindex under shopifypreview.com? This is the code I found in preview site, but not on live site.   Why? I am working on logic to set noindex to certain pages,of course I need to run it fi...

The other solution - it works as confirmed if you have the filters in the URLs:   {% if template contains 'collection' and current_tags %} <meta name="robots" content="noindex" /> <link rel="canonical" href="{{ shop.url }}{{ collection.url }}" /> {% ...

This works! This should be in the library! 

Same here - is there a way to locate the filter and set rules? So far I can use metafield with noindex set to false or true, but I don't want to filter pages to be indexed.  

See this thread:https://community.shopify.com/c/technical-q-a/what-drives-this-script/td-p/659748 Solution at the end.