Get variables of process start event

[ilansch]

Hi, I am making REST API call to start process instance by definition id, The parameters are processDefintionId and variables.
I want to validate the variables the user has sent, To do so, I need to query the process and get variables for the "Start Event", After i'm getting the required variables - i will check if he sent them in the request.

How can i get variables for start event ?

Thanks

[ilansch]

Solved.
Used REST API to get Form properties, then instead of Task ID and use Process Defintion Id