How do I make the values appearing in one dropdown for a choice column dependent on another?

Hi all,

Hope you're well. I have an issue on my SharePoint List Form where I'm trying to make the values that appear in a dropdown from one choice column dependent on another. For example, if the user selects Internal in field 1 of the form (which is a choice columns), then in field 2, on the dropdown for that field, only 3 values (A, B and C) appear. 

But if they select External in field 1, then when they go to fill out field 2, only D, E and F appear in the dropdown.

I've created a PowerApp that performs this function but I don't know how to make this function appear in the form over at the Sharepoint List, as that's what my colleagues are using. I want this to appear when they're filling out that Sharepoint List form.

Could anyone help me in this? Thanks in advance