Header set Link?

Hi,

I'm using httpd-2.4.62 on fedora40 and I would like to dynamically add a 
Canonical Tag to the HTTP Header. I set this in my .htaccess in the 
document root.

SetEnvIf Request_URI "(.+)\.pdf$" PDF_CANONICAL_URL=$1\.pdf
Header add Link "<https://%{HTTP_HOST}e%{REDIRECT_PDF_CANONICAL_URL}e>; 
rel=\"canonical\"" env=REDIRECT_PDF_CANONICAL_URL

However, the %{HTTP_HOST} variable doesn't seem to be set.

Using "curl -I" displays this:
Link: 
<https://(null)/images/joomgallery/details/gallery_2/img-1512_20210924_1725768871.jpg>; 
rel="canonical"

I followed the example here:
https://stackoverflow.com/questions/63768450/howto-add-canonical-tag-to-the-http-headers-for-dynamically-generated-pdf

Can you explain where the REDIRECT_ is defined? According to the info at 
the stackoverflow link above, it appears to be a fixed variable that 
somehow prevents the header above from being duplicated during the 
second pass, but I don't fully understand.

Is this the best way to do it? What if I wanted to move it to my apache 
config instead of .htaccess? I tried moving it into the VirtualHost 
config and it doesn't work. Maybe it's getting lost in the .htaccess 
scanning?

What am I missing?