How can i keep sperm from drying out if i am collecting in a sealed sterile cup for at home insemination?

You don't. Laboratories and research centers have specialized containers and transport systems that permit storage. Home storage would seldom if ever be adequate to sustain sperm for more than a brief period. Most of those kept in the listed way would exhaust their energy reserves and be incapable of normal function.

Insemination has to be done right after collection otherwise it will clot and then dryout. Diluting it with any additive will reduce sperm concentration.