Set a cookie if the URL contains specific UTM parameters.

function setUTMCookie() {

    // Search the URL

    var url = window.location.search;

    // Store the UTMs we're searching for in a variable - update the UTM value based on the UTM parameter(s) you want to save as a cookie

    var emailCampaignUTMs = '?utm_medium=email';

    // Calculate the expiration date for our cookie (1 year from the date it's set)

    var cookieDate = new Date;

    cookieDate.setFullYear(cookieDate.getFullYear() + 1);

    // If the URL contains the UTMs we're searching for, set a custom cookie named 'omCustomCookie' and store the UTM sstring as the cookie value, and expire it one year from the date it is set

    if (url.indexOf(emailCampaignUTMs) !== -1)

        document.cookie = 'omCustomCookie=' + emailCampaignUTMs + '; expires=' + cookieDate.toGMTString() + ';';

}

// Run our cookie function

setUTMCookie();