Click on: http://stevespages.com/ipb-taurus-pt145.html
(No, I'm not that Steve.)
The following is pure conjecture, as I have never taken one of these guns apart. I am going by the parts drawing I linked to, above.
The slide catch spring (#20) has a round bend and two legs. One leg has a little prong.
The round bend goes toward the front of the gun. The leg without the prong is at the top, and presses upward against an inward-facing projection* at the top of the slide catch (#23). The leg with the little prong is at the bottom, and the prong goes into a hole in the metal frame (#29). The spring may have to go between the slide catch and the metal frame, so the slide catch may have to come off before the spring can be replaced*.
*This is where the conjecture comes in. The spring (#20) may actually go between the plastic grip-frame (#34) and the metal frame (#29), and lie on the outside of the slide catch (#23). In that case, the spring leg without the prong probably presses upward against the thumb-piece of the slide catch.