When you create a project from a proposal, PPM automaticly relates both.
You can also add any request type: proposals, assets, other projects, defects etc; to the references. When add a thing to references, the reletion can see both things' reference tab. If add a proposal to project as refrence, you can see project in proposal reference tab.
PS: If the post resolves your issue, please click KUDOS star icon and mark it as Accepted Solution.
IT Governance Professional & PPM Solution Architect