Retrieve Plugin not getting triggered

We are on Dynamics CRM 2016 On-Premise. Using a plugin I'm trying to automatically update a field when a user open the CRM Account form, in this example to value "5". Here's my code:

 var targetEntity = (Entity)context.OutputParameters["BusinessEntity"];

 if (targetEntity == null)
throw new InvalidPluginExecutionException(OperationStatus.Failed, "Target Entity cannot be null");

var serviceFactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
var service = serviceFactory.CreateOrganizationService(context.UserId);

if (targetEntity.Attributes.Contains("MyField"))
fedTaxId = targetEntity.Attributes["MyField"].ToString();

targetEntity.Attributes.Add("MyField"d, "5");
targetEntity["MyField"] = "5";
service.Update(targetEntity); 

I list this in message type 10 (Before Main Operation Outside Transaction). In Plugin Registration I list this as Post Operation stage and Synchronous.

However when I open the Account form, the page blinks once but the value did not get automatically populated. There is no javascript that would've manipulated this form or value either. Any suggestion? Thanks.

Two options:

1. Add a script on the form setting the field value on load. Keep in mind this script should only do its thing if the form type = 2.

2. (Not recommended) Register a plugin on the synchronous post retrieve message for the entity. Make sure this step sets the field value on the entity object in the OutputParameters collection. Now, keep in mind your form will not be aware of the fact that this field has been modified, so it will not be flagged dirty and it will not automatically be submitted when record changes are being saved. So, in this scenario you would still need to add some JavaScript OR you would need an extra plugin registered on the pre update message of the entity setting the field as desired.