Random point within spline or layer

[Genete]

Not possible in a direct way (using a particle generator that does that) but you can have some alternatives that could be valid.
First you need to have a particle generator. There is a particle template I made some time ago that still working. It is on the wiki.

wiki.synfig.org/wiki/Particles_V2.0

Also, there are people working to automate this to be able to generate more than one particle type on one animation:

Particle exporter and firework example

Hello there! I am going to share some of my experience with particle template and that’ll result in a long post. So brace yourself Well… I was trying to achieve a simple task - make a firework with Synfig. Let’s see what we need to get this task done: We have to draw some firework rockets. More is better, because what a firework with only one rocket? It’s a lame firework. The rockets should throw away some neat particles while they fly. It’s not necessary , but it will make firework l…

Finally, you can use those and limit its scope to the bounding box of the shape you want to put particles over. Then finally only need to group the particles and put them above the shape with the “Onto” blend mode.

-G