you have to be careful here. pre-emergents prevent the germination of annual weed seeds. if you broadcast a product like treflan over your bed, the seeds you subsequently sow will likely not come up. there are two ways to overcome this:

broadcast your weed preventer, then create furrows or individual holes for your seed and fill them with bagged seed-starter mix. plant your seeds in the mix.

or

broadcast your weed preventer and germinate your seeds in containers or flats. once the seeds have sprouted the pre-emergent shouldn't harm the transplanted seedlings. it's a lot more work, though.