If you want to prevent projects from being automatically created, you could consider removing the Create Projects permission from the user for which the token used for analysis was generated. This would probably require that you have a technical user (with Execute Analysis permissions) that you provision analysis tokens with
Otherwise, the only way to add a project to an existing monorepo is through the UI. There’s no analysis parameter that can be set to do the job.
If you think something should change here, I’d encourage you to post in the following category (or I can move your post):