sharepoint online – How can i find the Lookup Column id when i pass the data to REST API?

As shown in some examples I must use lookup column ID when i pass data to Rest API. But I cannot find what the lookup column’s id. So How can i add this id to data in rest api?

JSON.stringify({ '__metadata': { 'type': 'SP.Data.FeaturesListItem' }, 
  'SubsystemId':"It will be integer"})

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