Currently, the application method is set globally by the admin. In our use case we have job posters who want to get applications through the email form, while others would prefer to send applicants directly to their HR website.
Therefore, I suggest to let job posters choose the application method per job offer.
Welcome to our forum.
The feature you request also possible to implement. you can add a custom input field under job submission form via action hook, please see the details on the documentation how you can add/remove validated custom input field on job submission form.
And then based on the input field selection you can filter application methods by filter hook. you can get custom field value by get_post_meta() and get the job id by get_the_id() functions
I hope you understand.