I don't think there is anything bad in placing your referal link on your posts provided the link isn't going to cause any bugs or damages either by hacking or stealing the clicker's data or information or Money. If you aren't placing referal link with the purpose of harming other users, you are free to post the link since you just want to earn from an airdrop or affiliate marketing.
You made mention of plagiarism and spamming as something that has to do with posting referal links. Actually, plagiarism is when you copy and paste content or steal content without the user consent and without the user's reference. This has nothing to do with sharing your referal links. Steem cleaners and cheetah are always responsible to detect and flag anyone who is found of plagiarism. This implies that you must engage in plagiarism and then post your referal link at the end of the post because you may end up getting detected by cheetah.
All you need to do is to write about the tokens or cryptos by making a very lovely and persuasive posts such that it will attract every user to be part of it. You can then post your referal link at the end of the post so that you earn your token as people make use of your referal link to register. I suggest you can adopt some code to write your referal such that the website doesn't have show on your blog just by encoding it with words like "click here to sign up". Once they click on the link, they automatically get redirected to that site.
I have seen most top Steemit users engaging in airdrop and they paste their referal link sometimes in encoded form. The funniest part about this is that most of these people make use of bid bots just to get more audience to view their blog and register with their links as this add more to their income. Some will even promise you some amount of steem or SBD just by registering with their links.
In conclusion, there is nothing wrong in posting your referal link at the end of your blog provided you don't have the intention to harm other Steemit users. This is one of the best ways to use Steemit because it helps you promote your business and this can be observed in people that engage in Affiliate Marketing, SEO writing, Airdrop Referrals and others.
Thanks for reading and I hope this helps.
As long as the nature of the link is fully disclosed I don't see how adding one to a post you authored, properly researched, credited and cited your sources, could be considered spam or plagiarism.
However, if the purpose of the post is to shill you links, then yeah, it might fit under either one of abuse categories.
Throw in some votes from bid bots and self voting with 200 accounts,
I will find you and I will give you a Brand of @SteemFlagRewards.
For those curious,
SFR is a Abuse Fighting Project based on manual curstion which provides possitive ROI on every approved by Moderator flag.
We seek abusive content and flag it.
SFR collaborates with various abuse fighting projects.
If the post is just the referral link then yes it would be considered spam.
To put a referral link at the end of your post. (post being on a random topic and not about the referral site) This is not considered as spam but more of an advertisement on your post. Generally referral links on post are belonging to the person posting.
One thing to be aware of when placing referral links, Try ensure they are not to any scams that might come up from time to time, especially when the price of Steem is high.
No one will chase after you for putting a referral link in your profile. Validate the source and do some research on the links too before adding, People may ask questions about the link and it re-assures them if you can provide answers on a basic level if not all the tech stuff.
The referral link is simply just networking your post and time. Expanding who will see the things you advertise through your post.