Action:

Send an HTTP request to SharePoint

Endpoint:

_api/web/fields('FIELD_ID')

Headers:

{
     "Content-Type": "application/json;odata=verbose",
     "IF-MATCH": "*",
     "X-Http-Method": "PATCH"
}

Body:

{
     '__metadata': { 'type': 'SP.FieldLookup' },
     'Group': 'YOUR GROUP NAME'
}