Hello Boris,
I’ve always done logins through microflows and Java – couldn’t find any login endpoints in the client API either.
Here’s the documentation for java logins (https://apidocs.rnd.mendix.com/8/runtime/com/mendix/core/Core.html#login(java.lang.String,java.lang.String,java.lang.String)) you can put it in a java action which you can then wrap in a microflow and reference/action that from your widget.
Kind regards,
Dragos