How to use a SQL Server Stored Proc (with no parameters) as a datasource ?

Yes, you can use a stored procedure as "datasource" IF the stored procedure returns a table. I have created this simple test and it works:

CREATE TABLE TableTest
(
	NAME VARCHAR(100),
	AGE INT
)

INSERT INTO TableTest
VALUES
('Jonh Doe', 20),
('Tom Wayne', 35),
('Tony Clark', 40)

SELECT * FROM TableTest

CREATE PROCEDURE ProcedureTest
AS 
BEGIN
	SELECT * FROM TableTest
END

EXEC ProcedureTest

If I execute this SQL code on SQL Server Management Studio it will return 3 lines with 2 columns containing the content of TableTest.

If you go to Power BI and go to "Get Data", choose "SQL Server Database" and put the IP, click on bottom button to "Advanced Options" and put EXEC ProcedureTest in the field for "SQL Instruction" it will load and show the return of procedure, if the return is a table, you will see a table. Like the following: