I setup the parameter "DNSRecord2Remove" but the custom command builder appears to force me to select an "Object Type" Is there a way to get around this?
If you want users to manually enter the required data, use a parameter of the Edit box type. For details about using parameters, have a look at step 5 of the following tutorial: https://www.adaxes.com/help/CreateCustomCommand. As for the type of object for the custom command itself, there is no way not to specify it as commands are executed against specific objects only. As a solution, you can create a custom command for the User object type and then create a Web interface action for it. The action should be always executed for the currently logged on user.
Also, the Powershell applet RemoveDnsResourceRecord is not recognized when running in ADAXES. Can I just install the applet on the ADAXES server using powershell?
Yes, you just need to install the corresponding PowerShell module on the computer where Adaxes service runs. If you have multiple instances of Adaxes service sharing common configuration, the module should be installed on all the corresponding computers.