When using Canvas Plagiarism Framework, the Canvas administrator (if not you) can add a similarity pledge to the account by accessing the Canvas administrator settings and scrolling to the bottom of the page. The option can be found under the Similarity Detection Platform header, titled Similarity Pledge
Here, the feature can be enabled and the wording edited if needed. This will then appear for every student before they submit, asking that they tick a box to confirm that their work is their own.
Note: Once enabled, this cannot be turned off.