I wish I could help. Maybe mix in some very fine grit or sand...
I had some begonia seed pods that I ended up crumpling into the container. Honestly, I didn't see ANY signs of seeds when I did that. But as I look into the container, I am definitely seeing some tiny little cotoyledons.
BTW, I sowed them in a clear plastic kitty litter container that I set above a radiator for heat and humidity. I'm beginning to think that size of the container and the volume of planting medium matters when you don't have a greenhouse.

Sheila

Fill, level and lightly firm the compost in your seed tray; place on the ground and water thoroughly with a fine rose on the watering can; dry the rim of the seed tray with a cloth or kitchen towel (nothing worse than sowing begonias with wet fingers), even though you don't actually touch the seed. Now, sieve a thin layer of fine compost or sand over the surface (just to fill in the pits of the seed compost.), lightly press. Take a sheet of white paper creased down the middle and place on it a small quantity of perfectly dry fine sand; onto this place a small quantity of begonia seed from your packet and stir them together with a dry implement (the tip of your knife blade, the tip of a plastic label etc.
Hold the paper over the seed tray and tap it with a finger to slowly move the sand/seed down until it spills over the end and onto the compost spreading as evenly as you can.
If this procedure is new to you it is a good plan to practice on another tray using the dry sand alone. Learn how to hold the paper in the fingers of one hand and tap with a finger of the same hand to move the sand gently down the crease in the paper.
Don't cover the seed with any compost; place the tray in a close environment and cover with a sheet of glass or perspex and then darken off. Darkening off is not essential but is my preference. Germinate at around 70F. Very carefully turn the glass daily wiping off the moisture collected on the underside. Once germinated remove to the daylight and a temperature of around 65F. Prick off into trays of standard seed or first potting compost as soon as large enough even if it means on the point of a knife.

In the region of 60,000 seed per gram.

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Save yourself a great deal of grief and do the following:

take your "soil" and put into a container that you can put into the microwave. Dampen it, using hot water if it is very dry. Put into microwave and "cook" it like you would any food. Depending on the volumn of soil, cook until you bring the soil temperature up to about 180 degrees (check it with a thermometer. DO NOT PUT THE THERMOMETER INTO THE MICROWAVE !!! Check the temperature outside of the microwave.) Allow to cool before using. This should prevent any damping off. It is worth doing. I concur with the other information above. HAPPY GARDENING!

take your "soil" and put into a container that you can put into the microwave.

This is not good advice IMHO. There are plenty of sterile mixes available that meet the requirements of begonias and other seeds. Jiffy mix is just one. Clean your germinating tray with a light bleach/water mix (10 parts water to one part bleach soaked for an hour). Boil your water that you will use to water the media if you think it is necessary. In my 15 years of growing these, I have never had a problem with damping off.

This article contain two methods.

Otherwise just cut the small envelope (inside the main envelope) completely open and tap it over the pot. Let the seeds fly down and land on the surface. YOu can always sperate later

Here is a link that might be useful: begonia sowing article