XRM WebAPI Create new record using JavaScript in model driven app form command button

Here is a sample code which creates a child record from a command button on parent table form. This code is called from command button so you need to pass Primary Control parameter to javascript.

function createPrescription(formContext)
{
	var currentDAID =  formContext.data.entity.getId();
    currentDAID = currentDAID.replace("{","").replace("}","");
    alert(currentDAID);
	
	var data =
    {
        "lifeline_name": "Prescription",
		"lifeline_issuedate" : new Date(),
		"lifeline_DoctorAppointment@odata.bind": "lifeline_doctorappointments("+currentDAID+")"	
    }
	
	Xrm.WebApi.createRecord("lifeline_prescription", data).then(
		function success(result) {
		 alert("Prescription created successfully.");
		},
		function (error) {
			console.log(error.message);
			// handle error conditions
		}
	);	
}

Hope it works.