Request Body for multipart/form-data from Collection Variables

I have an example for what I want in application/json

Here’s what I do for application/json

In Collection Folder Pre-request Script, I add this:

const loginRequest = {
    "Username": "test",
    "Password": "testpassword"
pm.collectionVariables.set("loginRequest", JSON.stringify(loginRequest));

And in the request body, I just choose raw type and put {{loginRequest}} in that.

Is it possible to do this for multipart/form-data?