You must have done something wrong, but I am partly to blame.
First, the finder itself has no focus at infinity. The focus is provided by your eye.
My drawing wasn't very accurate. I said three lenses, basically () () () but I drew only one (), because the principle is the same no matter how many lenses you use. Hopefully thisexample of a finder will make it clearer.
The parallel (collimated) light (1) comes from infinity, and is focused by the objective glass (OG) at fOG (2), and the eyepiece (EP) is placed so that its focal length (EP, fEP) is at the same focus as OG's.
From there, the EP relays the light by converting it again to a parallel (collimated) beam (3). The collimated beam then enters the observer's eye (OE), where it forms the image of the object at infinity on the retina.(4)
In optics if it works in one direction, it must work in the opposite direction, i.e. from 4 --->3--->2--->1. If you replace the eye with a light source (a pinhole) at the focus of a lens the light will be leave the lens as collimated (parallel) beam and enters the finder's EP, which is then focused at the common focus for OG and EP at (3), and proceeds to enter the OG from where it leaves as a collimated beam again.
If you have a flashlight with a fairly collimated beam then you don't an extra lens. You can just shine directly into the finder's eyepiece and that beam should exit at the objective end as a parallel beam. If there's a crosshair reticle at the OG/EP focus, the image of that reticle will be projected out and will be visible in another telescope if its eyepiece is at its infinity focus position. This is how you simulate an infinity source for collimaiton and focusing purposes.
You cna get small flahslights such as this one https://www.google.c...Qr4kDegUIARD3Aw with adjustable beam that's sufficiently parallel. Some are quite cheap.