ContactUs form create a class

I am not sure your requirement but as per above code snippet my understanding is , You need to implement one custom schedule task to delete records from Form that have empty value in "usermessage" field.

Yes, you can add this code snippet for delete records from form based on your logic.

Refer Scheduling custom tasks in Kentico 12 documentation.

It will be like this in the class?

using CMS.Scheduler;
using CMS.EventLog;

namespace Custom
{
    public class CustomTask : ITask
    {
        /// <summary>
        /// Executes the task.
        /// </summary>
        /// <param name="ti">Info object representing the scheduled task</param>
        public string Execute(TaskInfo ti)
        {
            string details = "Custom scheduled task executed. Task data: " + ti.TaskData;

            // Logs the execution of the task in the event log
            EventLogProvider.LogInformation("CustomTask", "Execute", details);

            // Returns a null value to indicate that the task executed successfully
            // Return an error message string with details in cases where the execution fails
            return null;

if (formObject != null)
{
    // Gets the class name of the 'ContactUs' form
    DataClassInfo formClass = DataClassInfoProvider.GetDataClassInfo(formObject.FormClassID);
    string formClassName = formClass.ClassName;

    // Loads all data records from the form that have an empty value in the 'UserMessage' field
    ObjectQuery<BizFormItem> data = BizFormItemProvider.GetItems(formClassName)
                                                            .WhereEmpty("UserMessage");

    // Loops through the form's data records
    foreach (BizFormItem item in data)
    {
        // Deletes all files stored in the form's fields
        BizFormInfoProvider.DeleteBizFormRecordFiles(formClass.ClassFormDefinition, item, SiteContext.CurrentSiteName);

        // Deletes the form record from the database
        item.Delete();
    }
}
        }
    }
}

To answer your question "Can I add the bellow code in a custom task?", yes you can add that as a custom task.

The question back to you is what do you want your custom task to do? You provide no information regarding what you want it to do.

I want a custom task to add it on Scheduling.