The second argument to the forEach callback is the array index. You can use that to index into values1, so you can multiply them.

values2.forEach((element, index) => 
    this.getField("Text8").value +=(element * values1[index] + '\n'));

getField is not a native JavaScript function. I'll just define it in the snippet below.

Some remarks:

• split('\r') may not be the best way to get the lines, as there might be \n characters in there too. I would instead match numbers with a regular expression ([\d.]+).
• As this split up produces strings, I would immediately convert them to numbers by applying the Number function to them.
• Instead of accumulating the result with +=, create an array with the products and then join these into a multi-line text to assign (once) to the field.
• Use the index you can get during the loop, and with that index get the other number to multiply with

var getField = document.getElementById.bind(document);

var collect = id => (this.getField(id).value.match(/[\d.]+/g) || []).map(Number);

var values1 = collect("Text6");
var values2 = collect("Text7");
this.getField("Text8").value = values2.map((element, i) =>  element*values1[i]).join("\n");

textarea {
  width: 5em;
  height: 150px;
  margin-right: 30px;
}

<textarea  id=Text6>
1
2
3
4
</textarea>
<textarea  id=Text7>
4
5
6
7
</textarea><textarea  id=Text8></textarea>