When you submit your application in the statutory questions it is clearly asked if you have had a visa rejection. If you answer no to it, knowing that you have had a visa rejected, you should inform IRCC.
All visa refusals should be disclosed. IF you are unaware of any, you can explain it in LoE. If you did not disclose it in your application, you can send a web form and inform IRCC of the same.
All your visa refusals should be declared in your application, not doing so can lead to misrepresentation. As per law, you have an obligation to be truthful. Section 16(1) of the IRPA states:
- 16 (1) A person who makes an application must answer truthfully all questions put to them for the purpose of the examination and must produce a visa and all relevant evidence and documents that the officer reasonably requires.
See Tuiran v. Canada (Citizenship and Immigration), 2018 FC 324 (CanLII), <
http://canlii.ca/t/hr59k
Alalami v. Canada (Citizenship and Immigration), 2018 FC 328 (CanLII), <
http://canlii.ca/t/hr6r1>