addBusinessHours.cls

global class addBusinessHours {
    global class request
    {
        @InvocableVariable (label='Start Date' required = true)
        global DateTime startDateTime;
        @InvocableVariable (label='Business Hours Id' required = true)
        global String businessHoursId;
        @InvocableVariable (label='Interval To Add (Milliseconds)' required = true)
        global Long intervalMilliseconds;
    }
    global class response
    {
        @InvocableVariable (label='Next Business Date')
        global DateTime updatedBusinessDate;
    }
    @InvocableMethod(label='Professor Flow | Add Business Hours' description='Adds an interval of time from a start Datetime traversing business hours only. Returns the result Datetime in the local time zone.')
    global static List<response> getUpdatedBusinessDate (List<request> inputParams)
    {
        List<response> responseList = new List<response>();
        DateTime updatedDate;
        if(inputParams != null && inputParams.size()> 0)
        {
            for(request pf : inputParams){
                updatedDate = BusinessHours.add(pf.businessHoursId, pf.startDateTime, pf.intervalMilliseconds);
                response rs = new response();
                rs.updatedBusinessDate = updatedDate;
                responseList.add(rs);
            }
        }
        return responseList;
    }
}

addBusinessHoursTest.cls

@isTest 
global class addBusinessHoursTest 
{
    @isTest static void check_NextBusinessDate() 
    {
        
        Profile sysadminprofile = [select Id, name from profile where Name='System administrator' limit 1];
        User adminUser = [select Id,Name from user where profileId= :sysadminprofile.Id and isactive=true limit 1];
        BusinessHours bhoursId = [select Id from BusinessHours where Name='Default' limit 1];
        System.runAs(adminUser)
        {
            List<addBusinessHours.request> inputparams = new List<addBusinessHours.request>();
            //record 1
            addBusinessHours.request pf = new addBusinessHours.request();
            pf.startDateTime = Datetime.now();
            pf.businessHoursId = bhoursId.Id;
            pf.intervalMilliseconds = 3600000;
            inputparams.add(pf);       
            Test.startTest();
            List<addBusinessHours.response> response = addBusinessHours.getUpdatedBusinessDate(inputparams);
            Test.stopTest();
            System.assertEquals(Datetime.now().date(),response[0].updatedBusinessDate.Date());
            
        }
    }	
}