You can sync your steps from your Polar device to Big Team Challenge via either Apple Health on iOS or Health Connect on Android.
iOS
If you have an iPhone, please follow the steps here to connect the Polar Flow app to Apple Health: https://support.polar.com/uk-en/support/connecting_polar_flow_with_apple_health
Next, connect Big Team Challenge to Apple Health by following the steps in this help article: https://help.bigteamchallenge.com/en/articles/152996-syncing-with-apple-health.
From that point onwards, your steps walked with your Polar device should sync to the challenge.
Android
If you have an Android phone, please follow the steps here to connect the Polar Flow app to Health Connect: https://support.polar.com/uk-en/flow-app-health-connect
Next, connect Big Team Challenge to Health Connect by following the steps in this help article: https://help.bigteamchallenge.com/en/articles/152960-syncing-with-health-connect
From that point onwards, your steps walked with your Polar device should sync to the challenge.