You can use WWW-Authenticate header to force basic authorisation on a endpoint.
e.g.
[{"id":"bb582f31.95be1","type":"http in","z":"bf9e1e33.030598","name":"","url":"/endpoint/path_a","method":"get","upload":false,"swaggerDoc":"","x":240,"y":2540,"wires":[["516d8812.ece38","767a31.47eafdd"]]},{"id":"516d8812.ece38","type":"debug","z":"bf9e1e33.030598","name":"","active":false,"tosidebar":true,"console":false,"tostatus":true,"complete":"true","targetType":"full","statusVal":"$base64decode($split($$.req.headers.authorization,\" \")[1])","statusType":"jsonata","x":340,"y":2600,"wires":[]},{"id":"767a31.47eafdd","type":"switch","z":"bf9e1e33.030598","name":"","property":"users","propertyType":"flow","rules":[{"t":"cont","v":"req.headers.authorization","vt":"msg"},{"t":"else"}],"checkall":"true","repair":false,"outputs":2,"x":430,"y":2540,"wires":[["4d1eeeb5.397f"],["11af2225.b4c606"]]},{"id":"11af2225.b4c606","type":"change","z":"bf9e1e33.030598","name":"send authentacate header","rules":[{"t":"set","p":"statusCode","pt":"msg","to":"401","tot":"str"},{"t":"set","p":"headers.WWW-Authenticate","pt":"msg","to":"basic","tot":"str"},{"t":"set","p":"payload","pt":"msg","to":"Not authorized","tot":"str"}],"action":"","property":"","from":"","to":"","reg":false,"x":570,"y":2600,"wires":[["4d1eeeb5.397f"]]},{"id":"4d1eeeb5.397f","type":"http response","z":"bf9e1e33.030598","name":"","statusCode":"","headers":{},"x":710,"y":2540,"wires":[]},{"id":"c9e4e387.950a3","type":"inject","z":"bf9e1e33.030598","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"admin:12345","payloadType":"str","x":270,"y":2720,"wires":[["96b03a0a.4a762"]]},{"id":"96b03a0a.4a762","type":"change","z":"bf9e1e33.030598","name":"","rules":[{"t":"set","p":"users","pt":"flow","to":"$append([$flowContext(\"users\")[$ != \"Basic \" & $base64encode($$.payload)]],[\"Basic \" & $base64encode($$.payload)])","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":480,"y":2720,"wires":[["ddbc374f.dad918"]]},{"id":"ddbc374f.dad918","type":"debug","z":"bf9e1e33.030598","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":670,"y":2700,"wires":[]}]
