Yeah it doesnt have to be fumarate, any salt version will do, so just use lemon juice to dissolve it.
In any case to turn it into fumarates if that is really what you want, you can dissolve fumaric acid in water (making FASW), then dissolve dmt freebase in that, and either drink it or evap for some dmt fumarate crystals (with excess fumaric acid but that shouldnt be a problem as long as you know your initial dmt dosage). Or to get cleaner dmt fumarate crystals, do FASA (dissolve in acetone, prepara FASA, add FASA to the dissolved dmt freebase, wait for crystallization and voilá
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But yeah if its just to drink, dissolve in lemon juice is easier.