I might suggest you take a look at how the Demand Assessments are meant to be triggered, assuming you're trying to use the OOB demand assessment to evaluate and score the demand for prioritization. (https://docs.servicenow.com/bundle/sandiego-it-business-management/page/product/planning-and-policy/concept/c_SettingUpDemandManagement.html)

Essentially, this happens OOB, when the the demand is in the Screening Stage. If there are no Stakeholders listed (in the related list at the bottom of the demand), the Demand Manager should get the assessment sent to them (unless the Assessment Required checkbox is unchecked on the Demand record itself). More users can be added to that list of recipients by adding Stakeholders (and ensuring that on the stakeholder record, Receive Assessment is set to true). These stakeholders can be added ad hoc, or added automatically by associating the stakeholders with a Portfolio, and then setting the Portfolio field on the Demand to the respective portfolio. Once that field is populated, and the record is saved, the Demand will inherit all Stakeholders from the Portfolio.