Return taxonomy ID

Hi all,

I started using this great plugin to create front end forms for post creation and editing. I have managed to grasp the basics but I am now stuck with something.
I have a taxonomy field group in my front end form displaying all the categories in my database. The idea is that the front end user selects one or more categories from the list. I want to then relate these categories to the newly created post using wp_insert_post.
However I cannot get this to work. I tried to follow this tutorial but I cant get the IDs.
https://www.advancedcustomfields.com/resources/taxonomy/

Thanks a lot,
Harry

https://www.ads-software.com/plugins/advanced-custom-fields/