How to configure Jira Cloud to send emails on behalf of our domain if we already using SendGrid

[Nicole Qu]

According this we should use subdomain if we already using SendGrid to send emails from our domain. 

• If you're already using SendGrid to send emails from your domain for another service, you will not be able to add that domain a second time. In this case, it is recommended that you use a subdomain for your 'Project Email Address' setting. For example, use Jira@notifications.company.com instead of Jira@company.com .

First of all, is there a workaround that we don't need to use subdomain?

Secondly, how to configure subdomain for 'project email address' setting?