How to create index on existing temp table.

CREATE TABLE #Users
    (
        ID          INT IDENTITY(1,1),
        UserID      INT,
        UserName    VARCHAR(50)
    )
    
    INSERT INTO #Users
    (
        UserID,
        UserName
    )   
    SELECT 
         UserID     = u.UserID
        ,UserName   = u.UserName
    FROM dbo.Users u
    
    CREATE CLUSTERED INDEX IDX_C_Users_UserID ON #Users(UserID)
    
    CREATE INDEX IDX_Users_UserName ON #Users(UserName)