I thought I saw a way to add your weight to the activity app. Is this true? And if yes how do I add it

The Activity .app will use your weight, if you enter it in the Apple Watch.app > My Watch of your iPhone:

See: https://help.apple.com/watch/#/apd3bf6d85a6

You can enter your birthdate, sex, height, weight, and wheelchair status in the Apple Watch app on iPhone. In the Apple Watch app, tap My Watch, then tap Health.

Hi

If you have not yet set up the Activity app, you will be prompted to enter your weight, along with other personal information, when first setting it up on your watch:

• When viewing the time on your watch face, tap on the Activity rings icon / complication, or;
• Go the Home screen (via a single press of the Digital Crown) and tap on the Activity app icon (coloured rings).

Afterwards, keeping your personal information up to date over time will help the Activity app and Workout app on Apple Watch to deliver accurate estimations.

To check and update any existing weight and other personal information:

• On your iPhone, in the Watch app, go to: My Watch > Health > Edit - tap and adjust items, then tap on Done.
• Or, on your iPhone, in the Health app, go to: Health Data (tab) > Body Measurements > Weight > to add a new measurement, tap on the "+" sign (upper-right).

More information:

Use the Activity app on your Apple Watch - Apple Support