Hi, epiqal
Welcome to Microsoft Community and thank you for sharing your concern with us today.
I understand that you want to know how to enter admin credentials for Remote Desktop Android client.
Click on the Desktop below:
Enter the IP address of the target computer in the first line of the figure below, and enter the user name in the second line as following:
Keep your phone and computer on the same local network before connecting.
I hope the information provided is useful for you. If you have any additional questions, please do not hesitate to reach out to our support.
Best Regards
Clara Geng | Microsoft Community Support Specialist