The 'Reply-To' Email Address is set based on the 'Default Incoming' checkbox in the required Email Account.
You must have an email address record created in the ‘Email Account’ DocType for the desired email address, for example, ‘abc@test.com'.
In the record for email address ‘abc@test.com' created in the 'Email Account' DocType, enable the 'Enable Incoming' checkbox .
Enable the 'Default Incoming' checkbox. This will then set this email as the email in 'Reply-To'.
For more information on Email Accounts, please click here.