ADFS Authentication with Multiple Forests for Remote Desktop Services

Hello,

How can I do ADFS Authentication with Multiple Forests for Remote Desktop Services?

I have an on-premises Remote Desktop environment and now we are merged with other company and they want to access our Remote Desktop Environment with their AD User through Active Directory Federation Services (ADFS).

They don't want to use a VPN tunnel for AD trust.

Is there any way, we can do ADFS Authentication with AD Forest trust for on-premises Remote Desktop environment? 

Thanks