Say whatever you feel. They will listen to you and sort it out into support or opposition to current legislative efforts. You don't have to have a perfect script - just tell them what you think. But I would caution you to talk about the issues you care about, not telling them you support a specific bill. Because they need to know what issues you care about so that when legislators propose changes to the bills (which always happens), they know whether those changes are aligned with what they are hearing from the people. You'll note that the letter OP linked to does not say that it supports the bill - it supports a specific change they want to be prioritized.

For clarity, the issue at hand is the “Big Beautiful Bill” does NOT reverse the tax treatment. The request here is to change the bill to reverse the current treatment.