I made a feature request to Affinity Designer, Vectornator and Motion:
Feature Request: Connection Line Tool
Please add a connection tool like in Keynote, Pages, Numbers, draw.io and various flowchart apps. It creates a connecting line between two objects. If one object is moved, that end of the connection moves too. In other words, you can anchor the end of a line to an object.
To try it in Keynote, select two objects (e.g. shapes or text boxes) then customize toolbar and add the Connect button, or use the menu: Insert → Line → Straight Connection Line (I added a hotkey for it in System Preferences). You can move either object and the connection line adjusts, you can style the connection line, and you can also drag the end of a connection line to attach to another object or unattach it and put it at a fixed position.