Ghost/ghost/admin/app/components/modal-upgrade-custom-integrations-host-limit.js

import ModalComponent from 'ghost-admin/components/modal-base';
import {inject as service} from '@ember/service';

export default ModalComponent.extend({
    router: service(),

    actions: {
        upgrade: function () {
            this.router.transitionTo('pro');
        }
    }
});