Which endpoint would I use to post a new record by API?

You can use this endpoint to submit the form:

https://api.formaloo.net/v3/form-displays/slug/{form_slug}/submit/

In order to submit a form, you should send the fields and their values in the body. For example:

{ 
"{field_1_slug}": "Some Text", // Text field 
"{field_2_slug}": 20, // Number field 
"{field_3_slug}": "{selected_choice_slug}", // Choice field 
"{field_4_slug}": [ "{selected_choice_1_slug}", "{selected_choice_2_slug}" ] // Multiple select field 
}

https://api.formaloo.com/docs/openapi/redoc/#operation/3_form_displays_slug_submit_create