Hi mohamed,
I've never used this component IdP React, but looking for the post you shared here, I understood that what you need to do is setting the site properties, and change the tenant you want before redirecting the user to the login page.
The TenantSwitch that they say in this post is an action from System:

This action receives a Tenant Id Parameter.
This parameter is the one that they are saying to add in the GetLoginURL action as Input, so this way they can pass it ahead to the Switch Tenant action and change the tenant before getting the URL.
Hope it helps,
Cheers and Regards,
RR :)