Date.prototype.ddmmyyyyhhmmss = function() {
    var yyyy     = this.getFullYear().toString();
    var mm         = (this.getMonth()+1).toString(); // getMonth() is zero-based
    var dd      = this.getDate().toString();
    var hh      = this.getHours().toString();
    var m          = this.getMinutes().toString();
    var ss      = this.getSeconds().toString();
    return (dd[1]?dd:"0"+dd[0]) + (mm[1]?mm:"0"+mm[0]) + yyyy + (hh[1]?hh:"0"+hh[0]) + (m[1]?m:"0"+m[0]) + (ss[1]?ss:"0"+ss[0]);
};
pm.sendRequest({
    url: pm.environment.get("base_url") + "/security/tokens?secretKey=" + btoa((new Date()).ddmmyyyyhhmmss()).reverse(),
    method: 'GET',
    header: {
        'Username': 'GTPSSP',
        'Password': 'gtpSSP4Ap1',
        'EnvironmentSettings' : '',
        'Content-Type': 'application/json',
    }
}, function (err, response) {
    var jsonData = response.json();
    pm.environment.set("accessToken", jsonData.AccessToken);
});