You can refer the below document
First you need to install and configure the Email Connector Marketplace Module.
The module is widely used and pretty self explanatory.
Once you configure it, see if it is working by using the Test Email Sending option (inside the EmailConnector provided page)
Now that you are ready with the EmailConnector do the below.
In the button that you mentioned in your question, make it call a Microflow.
This microflow should call the SendEmail Java action provided by the EmailConnector Module
You can find the details here: https://docs.mendix.com/appstore/connectors/email-connector/#41-sending-email
If you want to send email using an email template then, you can refer this: https://docs.mendix.com/appstore/connectors/email-connector/#43-using-email-templates
Let me know if it worked.