The command which needs the password should have the parameter defined as a 'Password'. The way to do this is to have an alias for the parameter in the command definition.
This is how you would define it in your command definition:
[parameter(Mandatory=$true, HelpMessage="Password of the user with privileges to add the SVM to the ADS domain.")]
When you use 'Discover Parameters', if the command parameter type is discovered as a 'Password', then you should be fine.
Also, the return parameter from the Get-WfaInputPassword is a SecureString type. You need to convert this to plain String before using it as an input parameter to AdminCredential in the cmdlet. Alternatively, you can use the variant that accepts the PSCredential as an input.
You can use the 'Create and Configure Virtual Storage Machine' command as a reference.