digraph mmm {
        abcdef   [ULR="http://yahoo.com?valueCDfield1 string",
                field1="field1 string"];
}

it should have %CD in the URL field from the two %% signs in the format to printf.
Input:

echo 'digraph mmm { abcdef [ field1="field1 string"]; }' | gvpr -c 'N { ULR = sprintf("http://yahoo.com?value%%CD%s", field1);}'