I do not think that there is an API endpoint for seeding in results for merge result pipelines. The API action for creating merge request pipelines allows to trigger a merged result pipeline, but there does not seem to be a functionality other than GitLab CI/CD itself.
Suggest creating a feature proposal, and add more configuration snippets to better understand the question (it took me a while to understand the relation to OpenAPI, merge request pipelines vs. merged result pipelines).