set any field value in lookup value instead of primary field value, in ms crm

//for example set opportunity amount instade of opportunity name 

function setOpportunityAmount() {

if(Xrm.Page.getAttribute("new_oppamount").getValue()==null){

return;

}

    var opportunity_id = Xrm.Page.getAttribute("new_oppamount").getValue()[0].id;

    opportunity_id = opportunity_id.replace("}", "").replace("{", "");

    get_OpprAmount(opportunity_id).then(function(result) {

        var new_oppamount = result["new_oppamount"];

            if (!!new_oppamount) {

            Xrm.Page.getAttribute("new_oppamount").setValue([{

                id: opportunity_id,

                name: new_oppamount,

                entityType: 'opportunity'

            }]);          

            }

        }

    ).catch(error => Alert.show(error));

   }

function get_OpprAmount(opportunity_id) {

    return new Promise(function(resolve, reject) {

        var req = new XMLHttpRequest();

        req.open("GET", Xrm.Page.context.getClientUrl() + "/api/data/v8.2/opportunities(" + opportunity_id + ")?$select=new_oppamount", true);

        req.setRequestHeader("OData-MaxVersion", "4.0");

        req.setRequestHeader("OData-Version", "4.0");

        req.setRequestHeader("Accept", "application/json");

        req.setRequestHeader("Content-Type", "application/json; charset=utf-8");

        req.setRequestHeader("Prefer", "odata.include-annotations=\"*\"");

        req.onreadystatechange = function() {

            if (this.readyState === 4) {

                req.onreadystatechange = null;

                if (this.status === 200) {

                    var result = JSON.parse(this.response);

                    resolve(result);

                } else {

                    reject(this.statusText);

                }

            }

        }

        req.send();

    });

}