@vikasverma1 

Just want to confirm

-> u_envalior_request_mapping table field u_requestor is reference to sys_user

-> you should use onChange catalog client script + GlideAjax to populate the string variable whenever that earlier variable changes

onChange Catalog Client Script: Requestor variable

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading) {
        return;
    }

    if (newValue == '')
        g_form.clearValue('stringVariable'); // give here string variable name

    if (oldValue != newValue) {
        var ga = new GlideAjax('ServiceCopilotAjaxUtils');
        ga.addParam('sysparm_name', "getRequest");
        ga.addParam('sysparm_userID', newValue);
        ga.getXMLAnswer(function(answer) {
            if (answer != '') {
                g_form.setValue('stringVariable', answer); // give here string variable name
            }
        });
    }
    //Type appropriate comment here, and begin script below

}

Enhanced Script Include: the function will work when called from server side (default value) + also from GlideAjax

var ServiceCopilotAjaxUtils = Class.create();
ServiceCopilotAjaxUtils.prototype = Object.extendsObject(AbstractAjaxProcessor, {

    getRequest: function(user) {
        var requester = this.getParameter('sysparm_userID') || user;
        var gr = new GlideRecord('u_envalior_request_mapping');
        gr.addEncodedQuery('u_catalog_item=Request Trial Copilot License^u_requestor=' + requester);
        gr.query();
        if (gr.next()) {
            return gr.u_requestor.getDisplayValue();
        }
        return '';
    },
    type: 'ServiceCopilotAjaxUtils'
});

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