https://www.michev.info/Blog/Post/3970/find-tenantid-by-domain-name-and-vice-versa-by-leveraging-the-graph-api

One interesting bit that caught my attention was the addition of the tenantInformation resource type and two methods related to it, namely: findTenantInformationByDomainName and findTenantInformationByTenantId. The first method can be used to quickly find out whether a given domain is in use within Microsoft 365 and which organization it belongs to, along with the tenant ID.

GET
https://graph.microsoft.com/beta/tenantRelationships/findTenantInformationByDomainName(domainName='office365itpros.com')\\
 
GET 
https://graph.microsoft.com/beta/tenantRelationships/findTenantInformationByTenantId(tenantId='b662313f-14fc-43a2-9a7a-d2e27f4f3478')