XF 2.0 Default avatar image with url that needs username?

[koa]

How can I quickly set default avatar's images to an Url that requires their username? For example:

https://someapi.com/username/${username}

 

[Mike]

This would require an add-on of at least some description.

 

[koa]

This would require an add-on of at least some description.

Could you lead me to the right place to find out more?