If that's the default situation (a likely scenario for most people), you'd need something other than a single code word.
In practice, most people can conduct a reasonable verification through a series of challenge/response interactions based on shared knowledge, should they need to do so. Mentioning something done, said, or shared in private recently would suffice in many instances.
For more robust tradecraft, should you need it, a set of one-time codes (passwords or passphrases) might substitute.
When the former head of InterPol was arrested in China, he managed to alert his wife through the use of a duress signal, an image of a knife:
In the film Capricorn One (1978), one of the astronauts alerted his wife by referring to a holiday they'd recently taken together, by mis-stating the destination as Disneyland, rather than Hollywood -- the land of make believe -- as it had actually been, which led to the revealing of the hoax mission.
If you can speak freely you don't need an extra codeword to explain that you are using the codeword in it's real meaning. Unless maybe you suspect that somebody is listening to you and might learn your codeword from that.
The codeword is to make it clear your words should be taken 100% seriously without considering the risk you are being coerced / spoofed with AI. If I agree on a word in advance with someone that no one could possibly guess and insert into an attempt to coerce / spoof my voice, then if there is truly an emergency in which I need this person to wire money to a random account, they will actually do it because they will know my request is genuine.
If I'm being coerced, I could have a codeword to indicate that. If I'm being spoofed with AI, I'm not in control of "my" words, so I can't. I need instead to prove when I'm not being spoofed with AI. That's the purpose of this second codeword.
While it's a great idea, how do you test this? Is it worth the time?
All my loved ones are on my iCloud so I would just ping their phone/watch while confirming location and asking the assailant to let you hear the phone ping on the line.
Sound like we'll all need more things like this eventually :(