I want to request a catalog item in vra 7.5 which have multiple page custom form using rest API's but not able to do so because /request/template API doesn't provide me with the fields defined in custom form. Furthermore, I can do it by defining custom form fields in custom properties but don't want to do that. Does anybody have a solution for this ??
You cannot request a catalog item using the custom forms through the API.
Custom Forms themselves are just a wrapper for requesting via the standard request template.
Whatever is running your API call should be doing everything programmatically or it has it's own UI (XaaS, vRO workflow, external script/service, etc.)