We implemented our own signup and creating users in Entra External Id via Microsoft Graph API. In the default signup of Entra, an OTP Mail will be created for the first sign in. Is there a way to trigger exactly this mail from the Graph Api?
To quickly answer your question - is it possible to send OTP email to the user using Microsoft Graph API when you have built your own registration process? No, it is not possible.
OTP code will only be send when you use either registration page provided by Microsoft Entra External ID or using the Native Authentication API.