esphome/tests/component_tests/api/test_homeassistant_variables.yaml

esphome:
  name: test
  on_boot:
    then:
      # Plain strings with a return statement compile as lambdas
      - homeassistant.event:
          event: esphome.test_event
          data_template:
            message: "{{ lambda_var }} {{ static_var }} {{ tagged_var }}"
          variables:
            lambda_var: |-
              return millis();
            static_var: static value
            tagged_var: !lambda return App.get_name();
            hint_var: id(test_sensor).state
      - homeassistant.action:
          action: notify.notify
          data_template:
            message: "{{ lambda_var }} {{ static_var }}"
          variables:
            lambda_var: |-
              return millis();
            static_var: static value

esp32:
  board: esp32dev

wifi:
  ssid: SomeNetwork
  password: SomePassword

api: