Hello,
how to correctly tag football tennis?
Football tennis on Wikipedia
According to taginfo:
football_tennis - 38
soccer_tennis - 4
futnet - 11
According to OSM wiki, the term football should only be used for American football and similar sports.
Soccer tennis is also good term, but I prefer futnet.
There is Union Internationale de Futnet - the world-governing body of futnet or Futnet World Championships.
No, letâs be clear: the wiki says that the word âfootballâ should not be used alone to describe any one particular sport. In English there are so many different sports regionally called âfootballâ that the tag sport=football would be so ambiguous that it would be functionally meaningless. More specific tags are used to elaborate, e.g. soccer, american_football, australian_football, canadian_football, gaelic_football, etc.
Iâve never heard of âfootball tennisâ, but I would simply go with whichever name for it is most commonly understood. You mention the name âfutnetâ, and the governing body uses the name âfutnetâ: is that the most commonly used and well-understood name? If so, go with sport=futnet. 
As âFutnetâ is the official name of the international union and the championship I would not hesitate to use this term instead of football tennis (which sounds awkward anyhow). Also Wikipedia writes
While the term football tennis is still commonly used, the word futnet is being used more as the new international name âŚ
When you start using it you could as well write a short wiki page for it to let others know.
Side note: As soccer is for a specific type of football soccer_tennis feels to be incorrect anyway? However wikipedia mentions it as alternative name with Football tennis as the primary.