Example for using the CrudField JS API

example.js

crud.field('owners').subfield('owner_id').onChange(function(subfield) {
    // TODO: do an AJAX call to an API you control;
    // the easiest would be to make a call to the FetchOperation on the same CrudController
    let ownerId = subfield.value;
    let theValueYouWantToSet = 'something';
  
    crud.field('owners').subfield('nick', subfield.rowNumber).input.value = theValueYouWantToSet;
 });