Hi in your client controller you need to do some thing like this. When they click reject button a pop up will appear to enter the comments and pass the comments to comments field.

add the below else part after        spUIActionsExecuter.executeFormAction(ESIGNATURE[state], "sysapproval_approver" , c.data.sys_id, [] , "", requestParams).then(function(response) {
                });
        } 

script:

else {
            
                var msg = '';
                var title = '';
                if(state == 'rejected'){
                    title = 'Rejection Comment';
                    msg = 'Comments are required when rejecting an approval';
                }
                spModal.open({
                    title: title,
                    message: msg,
                    input: true,
                    value: ""
                }).then(function(notes) {                    
                    c.server.get({state : state,comments:notes, update:true}).then(function(r) {
            });
                });
                }