In Spring 21 release, Saleforce provided an option to test custom domains on sandbox org which is generally available now according to the following link:
https://help.salesforce.com/articleView?id=release-notes.rn_security_domains_custom_domains_sandbox.htm&type=5&release=230

We checked the docs provided by SalesForce for setting up custom domain on Sandbox orgs. There is a step to add the domain on production org first and choose the org with which the domain will be linked. While setting up the custom domain in Production org, there is no field "Associated Org field" for selecting sandbox org. Can anyone please guide us on how we can setup a custom domain on our sandbox org? This is the link which we are following:

https://help.salesforce.com/articleView?id=sf.domain_mgmt_sandbox_custom_domains_setup.htm&type=5

In Spring 21 release, Saleforce provided an option to test custom domains on sandbox org which is generally available now according to the following link:
https://help.salesforce.com/articleView?id=release-notes.rn_security_domains_custom_domains_sandbox.htm&type=5&release=230

We checked the docs provided by SalesForce for setting up custom domain on Sandbox orgs. There is a step to add the domain on production org first and choose the org with which the domain will be linked. While setting up the custom domain in Production org, there is no field "Associated Org field" for selecting sandbox org. Can anyone please guide us on how we can setup a custom domain on our sandbox org? This is the link which we are following:

https://help.salesforce.com/articleView?id=sf.domain_mgmt_sandbox_custom_domains_setup.htm&type=5