We actually just did this.

Use case: On a form for HR/Finance where they need to see both what values (costing information for payroll) are currently set to and what they need to change to, we have a lookup that pulls the existing values into a table. The user then enters the new values into a second table. They needed a button that copies the values from the first table to the second (in the case where the values are not changing, or are only changing slightly).

In Classic designer, we accomplished this with a button that copies the values down to the second table.

In Modern designer, we are using a custom button to write a value to a hidden field, then use that hidden field value in Field Rules in the table cells. So, if hidden field = Yes, then table_cell_B = table_cell_A. We based this solution on this Answers post: 

https://answers.laserfiche.com/questions/210704/Clickable-custom-button-to-showhide-fields-in-Modern-Forms-designer

Here's the code we ended up with:

/* Set adjustable parameters for the form. */
const theFieldId = 48;

/* Listen for messages.  This allows this sandboxed */
/* iframe to receive messages from the form, */
/* so that custom buttons can be used to */
/* trigger this code. */
window.addEventListener('message', function (e) {
  console.log(e.origin);
  /* Handle the specific messages. */
  if(e.data.toString() == 'copy-button-clicked') {
    console.log('button was clicked');
    LFForm.setFieldValues({fieldId: theFieldId}, 'Yes');
  }
});

And here's the HTML we use in the button:

<button style="background-color: #C5BAD0;" onclick="document.getElementById('LFForm_sandbox').contentWindow.postMessage('copy-button-clicked', '*');">Click to Start the Test</button>

When the field value exists, we hide the button and show this "disabled" button in it's place (so the button can only be "clicked" once):

<button disabled="" onclick="document.getElementById('LFForm_sandbox').contentWindow.postMessage('copy-button-clicked', '*');">Click to Start the Test</button>

I hope that helps!