Your filter format looks fine. The List Campaigns API supports filtering by name using eq or sw, so filters=name eq "Exact Campaign Name" should work.
The next thing to check is authentication. The HTTP Request action doesn’t automatically inherit your tenant’s auth context. You’ll need to configure it with OAuth 2.0 Client Credentials Grant using the Client ID and Client Secret from a PAT. That PAT needs to belong to a user with ORG_ADMIN or CERT_ADMIN rights, since those are what the campaigns endpoint requires.
Also double-check that the quotes around the campaign name are straight quotes ("..."), not smart quotes. The workflow editor can sometimes auto-format those, and the API won’t recognize them.