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Thanks for the clarification. I was not aware of it. It will help me in the future. But, What if someone asks me to send a link via PM? Can I send it if someone asks to? In this case, I guess that guy was agreed to sign up via OP's link and then he send the PM. I am not sure. Hope OP will clarify this.
I believe it is okay to send a referral link if it is in agreement with the other party; the rules only apply to unsolicited PMs. For example, sending PMs to a large number of recipients you have not communicated with before is an absolute no-no.