Most targets are held by a cover or are crimped around the edge ‐ if yours is not that type it must be glued. The 'glue' used must be electrically conducting.
For 'professional' mounting use a few slivers of indium wire on the support plate, cover with the target and then either place in an oven with a flat weight on top of the target, or use a smoothing iron to heat the gold and hence melt the indium. Place something like a piece of lens tissue onto the target to protect it.
A temperature a little higher than melting point of indium is required (m.p. 156.6°C) for a short time.
Alternatively use a bit of silver conducting paint, preferably a few small drops of paste at the bottom of the jar.
You could also make a paste using our silver powder and a little commercial epoxy glue ‐ the method can be found here. Only spot-gluing is required, but keep a weight on the target while drying/ curing.