The highest-impact growth marketing tactics for a B2B SaaS startup with zero customers is often direct outreach. When you're at a point where you don't have customer validation, it's incredibly hard to get useful data from very small amounts of traffic. So in a zero customer state, your highest-impact effort is direct outreach.
Signing-up customers one-by-one is the best way to get customer validation and you'll learn a ton of valuable data along the way.
Once your customer validation is complete and you have at least a handful of customers *actively* using the product, then you have the rationale to start spending and experimenting with growth marketing tactics.
Happy to discuss the customer development process with you and share with you what has been successful for me in doing direct-outreach to enterprise customers.
Approaching non-competing businesses that already have your "avatar" and setting up some type of joint-partnership to encourage them to refer/recommend. Find people who have huge list already and give incentives.
Like Tom said, at zero you'll get the most value out of direct outreach to customers you believe are your target avatar. Ideally you'd do this before you build much software, in fact.
The best way to get the process rolling is to request feedback or an interview-like meeting with them to understand their business and problems, maybe doing a light demo of what you're building. Get them involved early and keep them updated as you progress, and when it comes time to ask them to try it out/sign up and pay you, you will have a warm lead and relationship.
Growth marketing is grunt work until you're scaling. Also happy to chat specifics if you'd like to set up a call.