https://tech.torico-corp.com/blog/bookmarklet/にあったコード

「フォーム内容を復元するブックマークレット」を作るブックマークレット

var out = "javascript:function sC(n,v,c){var elms=document.getElementsByTagName('input');for(ind in elms){o=elms[ind];if(o.name==n && o.value==v){o.checked=c;}}}function sI(n,v,c){var elms=document.getElementsByTagName('input');for(ind in elms){o=elms[ind];if(o.name==n){o.value=v;o.checked=c;}}}function sO(v){var elms=document.getElementsByTagName('option');for(ind in elms){o=elms[ind];if(o.value==v){o.selected='selected'}}}function sT(n,v){var elms=document.getElementsByTagName('textarea');for(ind in elms){o=elms[ind];if(o.name==n){o.value=v;}}}";

var elms, ind, o;
elms = document.getElementsByTagName('input');
for (ind in elms) {
    o = elms[ind];
    if (o.type == "hidden") {
        continue;
    } else if ((o.type == "checkbox" || o.type == "radio") && o.name && o.value) {
        out += "sC('" + o.name + "','" + encodeURIComponent(o.value) + "'," + o.checked + ");";
    } else if (o.name && o.value) {
        out += "sI('" + o.name + "','" + encodeURIComponent(o.value) + "');"
    }
}
elms = document.getElementsByTagName('option');
for (ind in elms) {
    o = elms[ind];
    if (o.selected) {
        out += "sO('" + o.value + "');"
    }
}
elms = document.getElementsByTagName('textarea');
for (ind in elms) {
    o = elms[ind];
    if (o.value) {
        out += "sT('" + o.name + "','" + encodeURIComponent(o.value.replace(/\s+/g, ' ')) + "');"
    }
}
out += "void(0);";
console.log(out);
var a = document.createElement('a');
a.href = out;
a.textContent = 'フォーム自動入力';
a.style.display = 'block';
a.style.position = 'fixed';
a.style.backgroundColor = '#eee';
a.style.padding = "40px";
a.style.top = "100px";
a.style.left = "100px";
a.style.zIndex = 1000;
a.style.border = "1px solid black";
document.body.appendChild(a);