I personally wouldn’t recommend passing domain admin credentials in a BigFix Task. The Bigfix agent runs as system on the local machine so normally doesn’t need separate credentials. If your have a specific use case where the install needs to run as a user you can use the override commands to change how the agent handles the deployment. There is also a way to execute the task as the local user but with system permissions, so that might accomplish what your looking for rather than passing domain admin credentials.
Here is a link to a post by Jason Walker regarding Override options.
https://forum.bigfix.com/t/tip-action-override-user-settings/40874