First PM in a company! I have not done it, nor have anyone in close network to have a good understanding. My guess is that they have to establish right roles/responsibilities on what to carve out from the CEO. Perhaps focused on scaling up product for next million users (or take on next set of enteprise clients), or execution focsed. Do take this with a grain of salt as I am guessing based on when should a CEO hire their first PM.
Misconception: B2B is about selling to businesses and so the end UI/UX is not as important.
In reality, the end user is always a person and there are people that are looking at easy products to use and making the decision to buy from you. The easier your product makes their job, the more inclined they will be to get your products. This applies for products as well as product operations - easy on-boarding, solid customer support and focus on data and reporting as possible.
I'm a fan of pairing. Often times Engineering works so closely with product, it is easy enough to sit more closely with your product partner and work through problems together. Ask questions and learn while on the job. If and when you get comfortable enough, start to volunteer to take on different tasks. I've worked with engineers who pair with me in writing requirements, buidling out tickets, reviewing designs, conducting user research. With this, you can now speak to first hand experiences and learnings within your existing company or any new ones you may be looking to move into.
The two disciplines are very different, despite some intersections on go-to-market, outbound communications, and occasional blurred lines between the roles in some companies regarding strategy and customer requirements. Early on in my career I had the opportunity to simultaneously work in both functions and experience them. Product Management has very broad scope and deals directly with technology. Ultimately, for me, building (or fixing) products felt most rewarding. :)
The biggest mistake I will mention here is not getting stakeholder buy-in. A lot of times we make assumptions about what people want and do not take the time ahead of to understand needs. Speak to those needs and share any constraints. Work to bring alignment across stakeholders. In the end, stakeholders lose trust in the product manager because they do not feel seen or heard. It is important that pre-work is done ahead of time so that when roadmaps are being presented, nothing comes as a surprise.
I'd love to answer this in a slightly different way: The single most important skill, that cannot be rated highly enough is Communication. Many other soft skills are fundamentally still rooted in or are dependent on communication.
Nuanced aspects of communication also matter: