How much salt (or any other substance one can find in a kitchen) do I need to add to make water boil at 104 °C?
According to [1, pp. 281–282], solution of sodium chloride $\ce{NaCl}$ prepared by dissolving 25 g of salt in 100 g of water has boiling point of 104.6 °C.
Additional data is available in the following table for the aqueous solutions of common salts and bases.
English transcription; column 1: Compound; columns 2–6: Concentration, g/100 g water — boiling points $(t_\mathrm{b.p.},~\pu{°C})$.




Note, however, that there is a significant drawback of using boiling salt solution due to the shift of boiling point upwards as the water boils off.
You either have to add water to the mark from time to time, or use a bath with another heat carrier and a thermal sensor.
References

V.A. Rabinovich and Z.Y. Havin. (Eds.) &quot;Kratkii khimicheskii spravochnik&quot; (Brief chemical handbook), 2nd ed., Khimiya, Leningrad, 1978. (in Russian)