With javacript, you can't directly include variables into the HTML. You have basically two options.
2) Perform everything with events calling functions
Example using #1 above
document.write('<INPUT type=hidden value=https://www.blahblah.com/info.html?variable='+var1+' name=rdr>');
The above examples assumes that var1 is global.
Example using #2 above
document.forms.rdr.value += '?variable=' + var1;
<form onSubmit="return setHidden()">
<INPUT type=hidden value=https://www.blahblah.com/info.html name=rdr>
Again, this example assumes var1 is global
Did I explain it all ok? Ask away if you have more questions.