Also, is the schedule field in https://airbyte-public-api-docs.s3.us-east-2.amazonaws.com/rapidoc-api-docs.html#post-/v1/connections/create the same as the sync frequency in the UI settings?

yes it should be

I set it as

"schedule": { 
    "units": 1,
    "timeUnit": "days"
  },

but in the UI it's showing "---"

Is it because it’s not defined here:

airbyte-webapp/src/data/FrequencyConfig.json

? cc @Artem Astapenko?

For the API, it looks valid though