Refresh Look up Field

 public void CalculateRollupFieldRequest(EntityReference target, string fieldName)
        {
            if (this.organizationService != null)
            {
                CalculateRollupFieldRequest calculateRollUpFieldRequest = new CalculateRollupFieldRequest
                {
                    Target = target,
                    FieldName = fieldName
                };
                this.organizationService.Execute(calculateRollUpFieldRequest);
            }
        }

/ <summary>
        /// Gets or sets a Input Argument - Opportunity Entity Reference
        /// </summary>
        [RequiredArgument]
        [Input("OpportunityEntityReference")]
        [ReferenceTarget(Opportunity.EntityLogicalName)]
        public InArgument<EntityReference> OpportunityEntityReference { get; set; }

 

        /// <summary>
        /// Gets or sets a Input Argument - Field Name
        /// </summary>
        [RequiredArgument]
        [Input("OpportunityRollUpFieldName")]
        public InArgument<string> OpportunityRollUpFieldName { get; set; }