Google said to use competitions to get links should be cautious
Recently, Google representative John Mueller warned webmasters that they should be cautious to use competitions to get links.
In particular, the problem of possible, if for participation in competition the link to contest is obligatory. In this case, the resulting links can be considered by Google as unnatural and lead to sanctions.
Hence the requirement to put a link to participate in the contest, is a violation of Google's recommendations for webmasters. Mueller explained:
The competition requires caution from the point of view of the purity of links. You should avoid situations when you ask for a link in exchange for something, or if participation in the contest a link to it mandatory
Listen to his answer, starting at 31:22 of the video below: