How can I override the AJAX template?

I would like to customize the AJAX template found in boltforms\templates\asset\_ajax.twig.

But how can I override the AJAX template with my own template inside my theme?

I have tried:

```
templates:
asset: my_sub_directory/_ajax.twig
```

```
templates:
ajax: my_sub_directory/_ajax.twig
```

But both options don't work. Overriding the form- or email- templates are working fine, using

```
templates:
form: my_sub_directory/form.twig
email: my_sub_directory/email.twig
```