Its hard to say from the image alone. It could be due to injury when planted or even weather conditions. Willows are susceptible to several diseases too, like cankers. I am not sure this is caused by a canker disease so you should keep an eye on it to see if it gets worse. Keeping the plant healthy to encourage vigorous growth will help. If you want to take precaution you can spray it with an approved fixed copper fungicide. 

If you would like further assistance, especially if the wound seems to worsen, I suggest taking clear, high resolution close up images and send them to the Tulsa County OSU Extension Center.

Tulsa County OSU Extension Center
4116 East 15th Street
Tulsa, Oklahoma 74112
Monday - Friday: 8 a.m. - 4:30 p.m.
Phone: <personal data hidden>

https://extension.okstate.edu/county/tulsa/horticulture.html