Access to data via the external API is only available on Custom Odoo pricing plans. Access to the external API is not available on One App Free or Standard plans. For more information visit the Odoo pricing page or reach out to your Customer Success Manager.

Source: https://www.odoo.com/documentation/18.0/developer/reference/external_api.html

So, yes, should be working.

Just ensure you have create a user account with a password (or API key) as explained in https://www.odoo.com/documentation/18.0/developer/reference/external_api.html#configuration 

Since you didn't describe what you want to achieve, you will have to refer to the examples in documentation linked above.

An implementation example for some use case can be found here as well: https://www.odoo.com/forum/1/280599#answer-280615