The Circle of Wildfire Druid subclass released in Tasha’s Cauldron of Everything has the feature Summon Wildfire Spirit, which (appropriately) summons a Wildfire Spirit. The feature includes this paragraph about what actions are allowed/required relating to the spirit:
In combat, the spirit shares your initiative count, but it takes its turn immediately after yours. The only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take another action. That action can be one in its stat block or some other action. If you are incapacitated, the spirit can take any action of its choice, not just Dodge.
However, sometimes characters can’t speak in order to issue commands, for example if the druid is bound and gagged I don’t see any restriction on summoning the spirit, but they would be unable to speak a command for it to help them escape. The Wildfire Spirit does not have any specific mention of a telepathic bond (though there is a vague "bound to your soul" line), but does specifically indicate it "understands the languages you speak". My assumption then was that in order to command the spirit it must hear you speak the command in a language.
However… Nothing in the feature actually SAYS any of those restrictions. There are at least a few instances where a creature explicitly "must be able to hear" something to have an effect, the first one I found being a Berserk Flesh Golem:
The golem’s creator, if within 60 feet of the berserk golem, can try to calm it by speaking firmly and persuasively. The golem must be able to hear its creator
Additionally, the (potentially) comparable Ranger’s Companion feature from the Beast Master Ranger subclass specifies that commands are verbal, and I would interpret that the beast would not be able to follow verbal commands that it doesn’t hear.
The Wildfire Druid feature does not include either of these phrasings, or anything similar. The spirit can "understand" languages you speak, does that include reading a written command? Could you attempt to convey meaning through gestures, or use telepathic communication from some other source? Is merely using your bonus action enough, even without an explanation for how the spirit would know your intentions (perhaps that would allow it to take a non-dodge action, but it wouldn’t necessarily take the specific action you wanted)? Or is "only verbal commands" the default and a special exception is needed for any alternative?