The CRA considers you to be in a common-law relationship if you have lived together with your partner for more than 12 consecutive months, or if you have a child together, either related to you by blood or through adoption, or if you have primary custody of a child under the age of 18.
You can still file your tax return as Common-Law and make the claim for the Eligible Dependant Amount in this first year of claiming Common-Law. You are entitled to make the claim because "at one time during the year" you met the CRA requirements, which can be viewed in this link: https://bit.ly/2oQPj5b
Here's some more information on Marital Status from TurboTax: https://bit.ly/2CqNmEp
