Help understanding hooks
First I think it helps to clear up some terminology... I've seen a few devs (myself included sometimes!) using the word &quot;hooks&quot; when they really mean &quot;events&quot;. For me at least this is a hangover from Craft 2 (and also ExpressionEngine), where &quot;hooks&quot; were used to inject custom back end code.
In Craft 3 however, everything is driven by events. Plugins/modules listen for these events and take action accordingly. (&quot;Hooks&quot; in Craft 3 are something different - more specifically, &quot;Template Hooks&quot; simply give you a chance to alter Craft's control panel templates and inject your own front end code).
Craft and the underlying Yii framework both fire hundreds of different events before and after every conceivable thing that happens, and likewise, plugins can do the same.
So to code a plugin/module that listens for events from Sprout Forms, you'd approach this in exactly the same way as a regular Craft plugin/module. Here's the general syntax for listening to Sprout's events: https://sprout.barrelstrengthdesign.com/docs/forms/events-and-hooks.html#events - in your case you'd probably want to listen for Sprout's EVENT_BEFORE_SAVE event and amend the POST data accordingly.