Hi Jessica,
That's a good point, so if you are in react and need multi-tenancy would say you need an approach like that, as you are suggesting:
Need to add there an extra parameter with the tenant in the idP login screen, and then also customize the preparation to do the tenant switch.
As add the tenant value itself in IdPReact on that function that you mentioned.
An alternative way to avoid customize both IdP and IdP react is you to have a traditional web screen on your end that do the tenant switch before you redirect the browser to the GetLoginURL.
Regards.