It will look like this :

export TYPESENSE_API_KEY=xyz

curl "<http://localhost:8108/collections>" \
       -X POST \
       -H "Content-Type: application/json" \
       -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
       -d '{
         "name": "user",
         "fields": [
           {"name": "username", "type": "string" },
           {"name": "db_id", "type": "string" },
         ],
         "default_sorting_field": ""
       }'
       
curl "<http://localhost:8108/collections/user/documents/import?action=create>" \
        -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
        -H "Content-Type: text/plain" \
        -X POST \
        -d '{"id": "1","username": "UserTest","db_id": "1"}
            {"id": "2","username": "UserTest1","db_id": "2"}'
            
curl "<http://localhost:8108/multi_search>" \
        -X POST \
        -H "Content-Type: application/json" \
        -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
        -d '{
          "searches": [
            {
              "collection": "user",
              "q": "UserTest",
              "query_by": "username,db_id"
            }
          ]
        }'

but with around 490k users with around 121 name near my expeceted result