Hi,
XaaS/vRA communicates with vRO via vRO REST API. At the moment, the type Credential is not support in vRO REST API, so you won't be able to pass value to workflow input parameters of this type when starting such workflows from applications that use vRO REST API.
One option is to modify the workflow and change its input parameters to not use type Credential. for example, such parameter can be replaced with a couple of new parameters, one for user name and one for password. You'll also have to change the scripting code in the modified workflow accordingly to handle the new input parameters.