Change default avatar to URL with custom field data?

[ScrapBlox]

I want to update the default avatar to a specific URL with data from custom fields.. I have a mandatory custom field on forum signup and I want to use that to get that way it sets the users default avatar to their game avatar (is this possible and how would I accomplish something like this?) (I don't want to overwrite all users avatars just those with no custom)

url format: {domain.tld}/u/{user_custom_field}

Change default avatar to URL with custom field data?

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