Obtain a value from a Suitelet form to populate into customer record (on create or edit)

Hi Everyone

I'm seeking a solution for an issue I have tried to source online but haven't had much luck.

Here's what I'm trying to achieve:

1. Add a button to customer record (via user event script or via inline HTML value which calls a function within the form's client script)
2. When the button is clicked, a popup window containing a suitelet assistant form will load
3. The user goes through the assistant steps and once submitted, they will be returned a value
4. The value is passed from the Suitelet back into the customer record and populated within a custom field

The part that I'm unfamiliar with is how to get the value back from the suitelet to be populated into the customer record's custom field.  I know how to use nlapiRequestURL via a client script to obtain information on a POST request Suitelet but have never tried to use a popup form where the user needs to go through steps to get the value.