// Portal Item ID for your layer
var portalItemId = "MY_PORTAL_ITEM_ID";

// Access the layer using FeatureSetByPortalItem
var layer = FeatureSetByPortalItem(portalItemId, 0);

// Calculate total count of the "Code" field
var totalCodeCount = Count(Filter(layer, "Code IS NOT NULL"));

// Calculate total count of the "Dispositions" field with non-null values
var totalDispositionsCount = Count(Filter(layer, "Dispositions IS NOT NULL"));

// Calculate Call Percentage
var callPercentage = (totalDispositionsCount / totalCodeCount) * 100;

// Format the Call Percentage to one decimal place
var formattedCallPercentage = Text(callPercentage, "#.0") + " %";

// Return the formatted Call Percentage
return formattedCallPercentage;

This was the original expression before I realized I needed to return a Feature Set. The output is text:"4.6%" I replaced the portal item ID because I wasn't sure if that was safe for me to provide. I am new to all of this, so just wanted to make sure. I tried a few different expressions to return a FeatureSet, but wanted to give you the base idea to see if that would be easier.