The Hebrew text of Psa. 104:35 states,
לה יִתַּמּוּ חַטָּאִים מִן הָאָרֶץ וּרְשָׁעִים עוֹד אֵינָם בָּרְכִי נַפְשִׁי אֶת יַהְוֶה הַלְלוּ יָהּ
The verb יִתַּמּוּ (yittammû) is conjugated in binyan Nifʿal, 3rd person, plural number, and imperfect tense from the lemma תָּמַם (tāmam), which would normally be translated into English as a passive voice verb using the future tense (e.g., “they shall be consumed”). Since it is placed in the lead position of the clause, it is acceptable (and perhaps, most appropriate) to translate it into English as a jussive, thus “let the X be consumed.”
Pratico and Van Pelt wrote,1
The basic form of the Jussive is the same as the Imperfect (3ms, 3fs, 3mp, 3fp). Like the Cohortative, the Jussive or volitional use of the third person Imperfect is identified by the position of the Imperfect verb in its clause. In general, when the third person Imperfect (not consecutive) verb is first in its clause, it is Jussive. Conversely, when the third person Imperfect verb is not first in its clause, it is not Jussive but the regular Imperfect (though there are exceptions).
Pratico, Gary D.; Van Pelt, Miles V. Basics of Biblical Hebrew Grammar. 2nd ed. Grand Rapids: Zondervan, 2007.
1 Ch. 18.14, p. 218