How in workflow to send email to address stored in a text field?

CRM 2011 on premise.

When we create a case we enter the email address of the person that reported the issue. We want to create a workflow to email them details of the case.

I want to specify the recipient as the value of the text field holding the email address of the person that reported the issue.

However, CRM will only let me set the recipient as an existing entity, such as the account email address, or the creator email address.

How can I send to the email address in the text field?