How to update form field after a button click.

In NetSuite transaction payment page, when customer clicks a button, it triggers a function in client script funct. The the function redirects process to suitelet post and get the process status retuned to client script function. All worked good as expected. Now, need to take response back data to update a transaction form field existing that was empty with the new value from Suitelet response. How can do this? What current doing is launch a dialog window with the data, then ask customer to manually copy it to the form field. It is not desired.

So, hoping Netsuite expert could recommend what do complete this? If there are some sample code to do so, that will be great. I am a newbie and just join a project to develop this process. Any recommendation and comments are welcome.