Not sure how Pocket handles it because they use a Chrome extension, but sites like Tumblr have "bookmarklets" that do this.
You basically have the user save a bookmark that contains Javascript, and that JS scrapes the page it's running on for the necessary content (headers, content, etc).
Here's the Tumblr bookmarklet for reference:
javascript:var d=document,w=window,e=w.getSelection,k=d.getSelection,x=d.selection,s=(e?e():(k)?k():(x?x.createRange().text:0)),f='tumblr.com/widgets/share/tool',l=d.location,e=encodeURIComponent,p='?url='+e(l.href) +'&title='+e(d.title) +'&selection='+e(s) +'&shareSource=bookmarklet',u=f+p,sw=0,sd;try{sd =d.createElement('div');sd.style.height ='100px';sd.style.width='100px';sd.style.overflow='scroll';d.body.appendChild(sd);sw=sd.offsetWidth-sd.clientWidth;d.body.removeChild(sd);}catch(z){};try{if(!/^(..)?tumblr[^.]$/.test(l.host))throw(0);tstbklt();}catch(z){a =function(){if(!w.open(u,'_blank','toolbar=0,resizable=0,status=1,scrollbars=1,width='+(540+sw)+',height=600'))l.href=u;};setTimeout(a,10);}void(0)
They grab the URL and title cleverly, and grab the user's text selection with a separate function, and create a popup form that auto-fills with the info.
Make sure you encode the script before adding as a bookmark (such as "=" would be "%3D").
I'm sure this code can translate to a Chrome extension, or easily Node, since it's JS based.