Well, it’s a matter of authentication, so the app needs to reference a certificate that’s known to Azure AD. See https://learn.microsoft.com/en-us/azure/active-directory/develop/howto-create-self-signed-certificate.

BTW, there are other ways to send email using the Graph. See this later article: https://practical365.com/send-email-powershell-graph/

AFAIK, the Graph API methods for sending email all depend on having credentials for a specific mailbox connected to a specific account. Work is ongoing to migrate EWS to the Graph and that’s when support for scenarios like sending email from a shared mailbox using SendAs permission will be supported. At least, that’s what I heard. Don’t quote me on it.

I have the shared mailbox where a service account has send-as permissions in Exchange Online.
I have the AAD app which has Mail.Send Delegated API permissions.

When I try to access token with MSAL.PS module, the option for client credentials is not there after I specify clientsecret etc.

Nope. I can’t debug code remotely. And advice given without seeing your code is unlikely to be much good.

I tried to use it by replacing the information, but the email was not delivered, nor did it return any errors.
Do you know what it could be?

I believe that this might be a gap that Microsoft has still to close. The current Graph APIs require a user identifier and don’t appear to support the Send As and Send On Behalf of scenarios for distribution groups and Microsoft 365 groups. I’ve asked Microsoft about this.