@hd_steve did you get this resolved?
You can’t. You have to create another API key.
If you want to keep the same value, you could copy the value, delete it, and then recreate with the same value and set the tenant you want.
There is no way to create API keys from the API.
You can bootstrap the system with an API key using Kickstart. Kickstart would allow you to create one or more API keys, which can optionally scoped to a tenant. More on Kickstart: https://fusionauth.io/docs/v1/tech/installation-guide/kickstart
However, this doesn't solve the issue of dynamically creating a tenant scoped API key. If that's a use case for which you need support, please file a github issue: https://github.com/fusionauth/fusionauth-issues with more detail.