010101 kishor There’s no super easy way. But, I made this old extension to add certain attributes to external links. See this line: https://github.com/zerosonesfun/elint/blob/ee072483808a185cad479b13ab95e54a5bbb3d99/extend.php#L37 That line adds the attributes like no follow / do follow / user generated content / etc. An option is try this extension but edit its extend file and adjust the attributes to your liking.
Wellwisher 010101 this is actually a really good mod. Great for SEO. Thank you. Typically you'd want all your internal links to be "follow" and external lists to be no follow. I like the "open external links in new tab" feature.
luceos One could also add this to the extend.php in the Flarum root folder (next to config.php). I think it would make sense to either have a generic "how should links work" extension or integrate something into core.