Custom avatars are not available in-game for players unless you win a contest (none are currently being held), and in that case ATA would contact you directly. Additionally, custom icons are unavailable in forums for anyone that is not a developer. To speak directly to a developer, you can send a help ticket by selecting the cog icon (⚙️) on your game home screen. It is located in the upper left corner. Then select 'Help' and begin a new ticket. However, as I stated, your request is not possible at this time so I wouldn't recommend contacting. But that is how you would do it.