Kindly follow the steps below to create the FAFR,
1. Access admin Module
2. Select the request template option and click the edit option the required request template the script needs to be applied on
3. Select the Fields and Forms Rule tab and paste the query then save
Query:
if(jQuery("#contactNoField").val() == ''){
alert("Please enter the phone number");
$CS.stopFormSubmission();
}
Added screen shot for reference