When you start D+ on the Roku, isn't it providing a way to log in? It's been long enough since I first installed it, so I can't remember the steps involved. When I added HBO Max last month, It offered a login screen that allowed my to enter my AT&T credentials. If I recall I had to complete the login on a web page, not on the Roku itself. Probably gave me a code to enter on a web page. It then linked to the Roku and away I went.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.