Hi @Adja Bineta 

We have used it in one of our requirement, i am sharing you the function that we have used in our onBefore transform script: Check it once if it will helps you,

 function raiseRequestUsingCartJS(userGR, source) {
        var requestJson = {
            "sysparm_id": "fa793f1n5k7654kbk47b4k91bfdb", //Catalog item
            "sysparm_quantity": "1",
            "variables": {
                'u_requested_for': userGR.sys_id.toString(),
                'u_employee_id': userGR.employee_number.toString(),
                'u_email': userGR.email.toString(),
                'u_requirement_business_justification': source.u_requirement.toString(),
                'u_valid_till': source.u_valid_till.toString()
            }
        };

        requestJson["variables"][pan] = true;

        var cart = new sn_sc.CartJS(gs.generateGUID());

        var cartDetails = cart.addToCart(requestJson);
        var rc = cart.checkoutCart();
        if (JSUtil.notNil(rc.request_id)) {
            // Updating the RITM value in the source record
            var grRITM = new GlideRecord('sc_req_item');
            if (grRITM.get('request', rc.request_id)) {
                source.u_ritm_sysid = grRITM.sys_id.toString();
            }
        }
    }

Please Mark this Helpful and Accepted Solution. If this Helps you to understand. This will help both the community and me..
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Thanks & Regards 
Deepak Sharma