public class getpara_Controller {

private ApexPages.StandardController sc;
public getpara_Controller(ApexPages.StandardController sc) {
    this.sc = sc;

}

public String para = ApexPages.currentPage().getParameters().get('custompara');

custom_object__c obj = new custom_object__c(lookup__c = para);

public PageReference insertRedirect() {

    sc.save();
    PageReference endpage = new PageReference('https://www.example.com');
   endpage.setRedirect(true);
    return endpage;
}

}