Yes, basically, here is what I would do:
1. Create a Data Element using Query String Parameter (and grabbed the passed ECID) - make sure this will default to no value if there is no passed ECID.
2. I would add a custom code rule that loads as soon as possible... maybe using the Library Loaded trigger, to ensure that the cookie is updated ASAP.
The code should look something like:
var newECID = _satellite.getVar('queryStringECID');
if (newECID){
var now = new Date();
var time = now.getTime();
var expireTime = time + 1000 * 60 * 60 * 24 * 730;
now.setTime(expireTime);
var cookieName = "s_ecid";
var cookieValue = "MCMID|" + newECID;
cookieValue = encodeURIComponent(cookieValue);
var cookieString = "";
cookieString = cookieName +'=' + cookieValue + ';expires=' + now.toGMTString() + ';path=/;domain=domain.com';
document.cookie = cookieString;
}
First I grab the value of the Data Element into a variable "newECID", then I check IF I have a value (otherwise don't overwrite what is there... we don't want to accidentally clear the value).
Make sure to reference the name of your Data Element (I used "queryStringECID" as a sample), and make sure to change "domain.com" in the cookie string to the correct domain where your s_ecid cookie is set (so that you overwrite the cookie, and don't create a second one on a different domain variant)
While you could use:
var newECID = _satellite.getVar('queryStringECID');
if (newECID){
document.cookie = "s_ecid=MCMID%7C" + newECID;
}
There is no guarantee this will set to the correct domain (particularly if your Shopify is using a subdomain, but the cookie is set to the main domain), so you might end up with two s_ecid cookies... but give it a try... it might work.
This should overwrite the ECID and now should send this visitor id on your tracking call, instead of the newly generated one...
