Conjugation is for verbs, but thankfully the answer takes both forms: The verb "to will" has meaning that is contained in various words in dovahzul, and not just one. Take a look at the thesaurus entry and it's multiple verb tabs: https://www.thesaurus.com/browse/will You will see that you have choices for wording.

Looking through that, you can find two known Dovahzul words there: laan and uth.

Zu'u laan (s), Mu laan (pl)

Hi laan (s, pl)

Rok/Rek/Nii laan (s),

Nust laan (pl) Because laan and uth can be nouns or verbs, you can also use endings for the noun forms to express what you command or want:

Laani/Dii laan, laanu/un laan (my want, our want)

Laanii (his/her/its want)

Laan do niin (their want)

You can replace laan with uth. For your final translation:

"Because I WILL it!" "Fah zu'u LAAN/UTH nii!" "Fah nii los LAANI/UTHI!"