I would like to create a form and receive the posted data in V9?

I need to manipulate some of the ViewModel fields before the form is displayed for some of the users.

I had thought using route Hijacking with a RenderController , creating the model and manipulating the form fields (this part works) and then posting back to a RenderController would be the way to go. However I'm having issues where the RenderController only sees the Hijacked Umbraco parent model and not my ViewModel. I've tried the suggestions on https://our.umbraco.com/Documentation/Reference/Routing/custom-controllers for V9

If this is the way to go does anyone have an example of the ViewModel constructor and RenderController post method?

Is there another way I should be going about this?

Disclosure: I had asked a much more specific question about this earlier but I thought it might be better to not be so specific - https://our.umbraco.com/forum/using-umbraco-and-getting-started/108056-post-form-model-with-route-hijacking-v9-cant-get-the-model-to-post