Click the Overflow (3-dot icon) on the Assignment card and click Manage. Alternatively, click on the Assignment card to open the Assignment, open the right menu and select Manage Assignment Settings.
On the Assignment Settings subpage, click the Others tab.

Select the checkbox beside Set as Assessment.

Note:

Assessment cards are differentiated with a beige background.
Students will not be able to access embedded links or apps in the Assignment unless they have been whitelisted for use in the Assessment's network.
Students will not be able to view Assessments on their Assignments tab on the homepage or the Assignments list. You will have to provide them with the Student Attempt URL for them to gain access to it by clicking the Overflow (3-dot icon) when inside an Assignment/Assessment.

Set Assignments as Assessments

Did You Know:

If you do not wish to create an Assessment (which will require the whitelisting of embedded links or apps), you may wish to set a Passcode to an Assignment. Students will require the Passcode before they can attempt the Assignment.

(D1,ii) Edit Quizzes (New)

(D1,iv) Add and View Passcodes