Supplier Portal: custom page built in PaaS and available to the supplier

Summary:

We are evaluating to develop a custom page in PaaS where the supplier will save specific informations required by the customer and upload attachments.

Once the data will be submitted, a custom workflow will be triggered in order to ask the approval to the company.

Is it possible to build a custom page in PaaS to be available to the supplier via Supplier Portal?

Content (required):

Supplier Portal

Version (include the version you are using, if applicable):

23A

Code Snippet (add any code snippets that support your topic, if applicable):