URL for medium seize image – similar to [field thumbnail-url]

How is it possible to link to a medium seize image – the same way you can do this by linking to the thumbnail URL: [field thumbnail-url]

I am trying to loop through a list of attachments and link with LightBox to the medium seize image (I removed here all unnecessary tags for simplicity):

[loop type=”attachment”]

<img src=”[field thumbnail-url]” />

[/loop]

I am looking for a syntax/workaround to create something like the [field thumbnail-url]

Many thanks for any help!

https://www.ads-software.com/plugins/custom-content-shortcode/