When you first login to the Manifold Developer Portal, you will see a dashboard containing all your Developer Applications. Developer Applications are needed to use the Manifold Client Widgets/Libraries, and the Manifold Server endpoints.
After signing up, select "Create New App" and enter an App Name and Description. Note that the App Name here will be referenced in wallet signature requests (e.g. "Please sign this to allow App Name access to...").
Configure the redirect URI to point to the website for your dApp and choose your Grant Type.
Click "Create" and your Manifold App will be created with a unique Client ID (and potentially a Client Secret, depending on the grant type), which will be used in configuring the client widgets/libraries and accessing the server-to-server APIs.