How can I record my weight using Siri?
It's not directly possible to say "Hey Siri. Record my weight as X pounds" currently (up to and including iOS 12).

However, there are two ways you can do this(-ish).


Siri Suggestions: if you launch the Health App (manually) and log your weight enough times, it will start showing up as a Siri Suggestion in Search. This will launch the Health App in the Weight section, and you can then log your weight manually.
Shortcuts: you can create a shortcut in the Shortcuts App. Use the Log Health Sample action, and select Body Measurements --> Weight. This will allow you to manually log your weight from a prompt. You can have it triggered by saying "Hey Siri. Record my weight". If you want you can make this more sophisticated with a combination of alerts/prompts and variables. It is not possible to launch a Shortcut and pass it input in the same statement (i.e. "Record my weight as X pounds", but you can launch the shortcut, and then have the shortcut use voice prompts and take voice input from you (note, you will have to convert text input to a number for the Health App). You will very quickly come to realise that Shortcuts is not a programming language, and gets clunky very fast. It's ugly (and fragile!), but it works (except when it doesn't)