Typescript Interface Add Unknown Dynamic variables

I'd like to define a interface that has some known variables and some other dynamic.

Let's assume this definition of interface:

export interface ApplicationData {
      applicant?: ApplicantData;
      application_unit?: ApplicationUnit;
      interested_people_count?: number | undefined;
}

How would I define that in this interface could also be:

interested_person_1: InterestedPerson;
interested_person_3: InterestedPerson;
expenses_1q_2023: Expenses;
expenses_3q_2022: Expenses;

So I do not know in advance how many these variables will be there, only what I know is that if it starts with 'interested_person_' then it is InterestedPerson interface and if it starts with 'expenses_' then it is Expenses interface.

We need to define two index signatures, one for interested persons and the other one for expenses. To define the keys we are going to use template strings.

Example:

type StartsWithInterest = `interest_${string}`

This type will handle all strings that start with interest_ and the rest of it doesn't matter for the compiler:

const ok: StartsWithInterest = 'interest_correct'
const notOk: StartsWithInterest = 'incorrect' //expected error

We can apply the template string to our index signatures that will give us the desired result:

type GenericType = {
  [x: `interested_person_${string}`]: InterestedPerson;
  [x: `expenses_${string}`]: Expenses;
};

playground