How to conduct market research?
How to properly and 100% legally conduct market research before starting development if I don't have any customers and their emails yet? Obviously I can't harvest emails and send surveys to those companies without their consent. How to do it then? Can i post a survey link on forum that is read by people from industry? Are there any other ways?
What type of product are you building? What problems does it solve? What type of people have those problems?
Those are the initial base questions to answer, from there you want to create 'buyer personas', get a sense of who these people are, their backgrounds, interests, what makes them tic, etc and the best way to do this is get out and talk to people.
You don't need to buy email addresses for this - if you're more comfortable staying online I'd recommend you join subreddits for your niche and read the posts, questions, and ideas people have - offer free advice and build relationships as well.
If you want to get out the door attend meetups for your niche, network with folks and get to know them, each interaction adds to your buyer personas. Once you can confidently say "our ideal customer is between x and y years old, works in such and such industry...ect" you've got a very good foundation for how to market your product.
Surveys are good too, but nothing beats face to face interaction and real conversations.