"The easiest way I can see would be to frame a helpful, curious question to which your service just happens to be the answer. So then you most an Ask HN like "can anyone help me to understand why people do X" followed by a few sentences of your thoughts, then at the end say "I've been working on a service to help with this but we don't seem to be getting much to traction, here's a link"."

Nah, that would be obvious marketing to most. More sneaky would be answering the question with the recommendation from a different account in a way to promote your service and have that upvoted, but that requires more effort and skill I assume.

But in general yes, this community definitely can also be manipulated, but I would say it is one of the hardest to fool. The standard mentality here I actually would rather describe as critical instead of curious, but there is just lots of garbage being pushed and also my curiosity is limited.