Using keyword comments in giveaway posts for SEO

I'm currently managing a client's reddit account. I mostly just create content for him to post, so not really clued up on SEO etc.

We regularly host giveaways in a large subreddit. Giveaway posts usually gain 1k to 2k comments.

If I made it a requirement that each entrant paste a keyword phrase within their comment, such as

Where can I buy ______?

I want to buy a ________ from [client website]

Then id have 1,000+ comments with those keywords.....Would that be helpful for SEO or link building?

Thanks in advance!

It can boost seo, however you will need to do multiple posts of that kind. Multiple comments with the same link will not improve things, however multiple posts containing those links in comments will.