You can use the Dashboard UI to set the content of the email templates by going to the Portal Management menu and selecting Settings. Here you will find fields for the welcome email (Welcome email customisation section - this is what you asking about) and also the API key approval email (API Key approval email section).
If you are using an on-premises installation then you will also need to configure your Dashboard so that it can send the emails. To do this, please check our portal email configuration documentation page. For Cloud and Hybrid, you do not need to worry about this since you are using our cloud platform and it is already configured.