public class getpara_Controller {

private ApexPages.StandardController sc;
public String para {get;set;}

public getpara_Controller(ApexPages.StandardController sc) {
    this.sc = sc;
    para = ApexPages.currentPage().getParameters().get('custompara');
    //This is where I try to set the field value, clearly the sc.save() is a separate save of a custom_object__c record from the vf page.
    custom_object__c obj = new custom_object__c(lookup__c = para);
}



public PageReference insertRedirect() {
    //Right here, this is where I want to assign the field value before using the sc.save()
    sc.save();
    PageReference endpage = new PageReference('https://www.example.com');
    endpage.setRedirect(true);
    return endpage;
}

}