As discussed in this forum post Using Arc on the IBM Cloud with Security Mode 5 you can use Arc with the IBM Cloud by uploading Arc to the Remote Desktop System. You can also use Arc from your laptop / desktop and connect remotely to the IBM Cloud via the REST API.
The key to connecting to the REST API is to have the correct URL for your TM1 instance, then added it to the settings.yml file like so:
REST API URL
The REST API that you need to use look like this:
You need to replace CUSTOMER-NAME with your IBM Cloud name and INSTANCE-NAME with the name of the TM1 instance you want to connect to. NOTE: The URL is case-sensitive, CUSTOMER-NAME is normally lower-case.
As discussed in this post Using Arc on the IBM Cloud with Security Mode 5 you need to use your TM1 automation credentials unless you change to security mode 1.