Is this even an apropriate question for meta? I dont know where else to ask, but I think arquade really needs a "Growtopia" tag.
I think after making a question you can flag it for moderator attention with the other reason and then just ask them to make the tag.
Apparently >300 rep users can make tags: https://gaming.stackexchange.com/help/privileges/create-tags
So as Robotnik mentioned, just leave a comment asking someone to create a tag.