How can I convince stakeholders and management that I need a user research focus group?

Background info on the company:

I'm the only User Experience Designer at a start-up-size company that has been around for 20 years. It's only been in the last few years that they've even created a development team rather than continue to sell white-labeled software.

Our product has End Users and end-end users. An example would be theme park employees. At Disneyland, there are characters in costumes, food service vendors and ride operators. These would be our end-end users. They check their schedule and clock in and out and take breaks and whatever. The people who manage those characters, vendors and operators are our End User, (like a Power User or Admin of sorts).

My role:

My 'voice' seems to count for a lot (and I've been outright told it does by my manager), because of my business and marketing knowledge, my background as a front end developer and general UX design experience. They realize they don't know what they don't know about the user experience and want an 'expert' to make the design choices that create a consistent experience. I follow a largely Lean UX-based process, and I advocate for the user where it makes the most sense while adhering to best business practices.

However, there are some instances where I feel like I need to weigh in and push through some objections from stakeholders.

A good example came yesterday, with a primary stakeholder on a project replying to a ticket about changing the wording on a button. It's a small change, and one that - apparently - clients have requested via the support team. The PM argued against reasonable responses from devs, and when I suggested we get feedback from customers who are actually using the product, his reply was "I don't know that our customers would give us the right answer – as they're not UX designers."

He's right, they're not UX designers, but they are end users of the product and we should be relying on their feedback to create the changes they want to see.

What I want:

I would like to create a user research focus group where I can connect directly with current clients to ask specific feedback about our UX choices and enhance their experience iteratively.

My question:

• What phrases should I use to express that a user experience can't only be defined by the designer, but that it also requires input from the users?

-or-

• What arguments have you used successfully to win access to user input?

I checked out this question, and found the selected answer helpful to an extent. Sales and support staff have great feedback. In this particular case, the feedback is being 'overruled' by the PM, so I need to overrule his balk to allowing me to connect with users.