Create a user approval through script in ServiceNow workflow

 
var approval_gr = new GlideRecord(‘sysapproval_approver’);
approval_gr.initialize();
approval_gr.state = ‘requested’;
approval_gr.approver = gs.getUserID();
approval_gr.sysapproval = current.sys_id;
approval_gr.insert();

The approver and sysapproval columns should be replaced according to your required values.