Where to promote the Landing Page?

First off: congrats on already having customers before launching. You are ahead of 95% of all indie hackers.

Move away of the mindset of "promoting". Instead be genuinely helpful to your audience. Yes, it takes time, but the returns are way better than simply dropping a link to your site. Your potential customers already get a lot of spam like this.

What can you do?

• write useful articles
• create free goodies they can use (checklist as pdf, poster to hand on their wall, etc.)
• create video's of saif articles
• answer questions in the communities the customer's decision-makers hang out (so unlikely this will be Indie Hackers or Product Hunt!)
• pull a small feature from your product and make it a free tool

I can go on, but you get the gist. Look at marketing at being useful to others instead of promoting at some point, you will get the rewards for it.

Also how did you the previous (promised) customers? Is this something you can replicate?

Building audience on twitter , participating on producthunt 's discussion page, building relations on indie hackers, these are some ways you can be a part of the community and also promote your product at the same time. Personal DMs on twitter also helps in building relations. Once you build relations they will also be sharing your product with the ones they think will be helpful for. you have to get active on all these platforms. Linkedin works too in this case. you have to build an engaged audience on all possible platforms. That's the only key.
Paid ads is a way to start to but that is very expensive and not sure how much your budget is for CAC. organically growing your followers/audience and converting them to customers is the ideal way. In such case you will also recieve feedback which i am sure you would need in the beginning.
Promoting your landing page where there is an audience to reach is the best way.
https://twitter.com/_girlintheframe thats my twitter handle let's connect :)

LinkedIn might be a great place to start. Start by sharing the LP on your profile and then ask people you know within your niche to share it on their pages.

Look for people on LinkedIn who are in your target niche and ask to become a connection. Don't promote the landing page in your first message to them! They will look at your profile and check out out LP before deciding to connect with you.

Product hunt is also a good site for things like this.

I also find the strategy of leaving valuable reddit comments and then dropping a self promo at the end helps.

For example, I'm developing an AI API that provides AI as a service to developers making AI apps. I usually find AI subreddits and comment under posts there, and then leave a self promo at the end of the comment.

Just make sure your comment is relevant though.

If you're curious to see what I'm doing with this, feel free to follow on twitter: https://twitter.com/TheRealEtch

Here's the website in case you're interested to see what type of self promo I do at the end of some of my comments in terms of content.

All Channels has its benefits. We need to see the placements where your target audiences focus more. In general, Google Organic (do SEO for keywords - get them in 1st page); Google Paid ads minimal for quick result but not for ROI, need streamlined process in place. Take some market research agency help. Socials posts, ads, link building to biz sites, local news, etc. Depends on your service and customer, choose the placements to be displayed. Post by https://apeiron.sg/

Posting my project on different communities worked best for me, especially Facebook and Reddit. Here is a list of communities that I use https://communitylist.me