// Set field to not required
crmForm.SetFieldReqLevel("fieldname", 0);
// Set field to business required
crmForm.SetFieldReqLevel("fieldname", 1);
// Set field to business recommended
crmForm.SetFieldReqLevel("fieldname", 2);
Update: new code placed based on the comment by Peter. Before the update I had this piece of code:
// set the field required (i.e. show error message when field is not filled in)
crmForm.all.fieldname.req = 2;
// modify the label to be red
crmForm.all.fieldname_c.className = 'req';