Javascript to input value into textbox instead of sendKeys() in Python

First, the correct way to set the current value of an input is to assign to the value property. There is no attribute for the inputs current value (the value attribute is the input's default value, more here).

The rest is a special case of a general-purpose question: "How do I output a Python variable's value into JavaScript code?"

If the string you're outputting doesn't contain quotes or backslashes, you may get away with using a format string and outputting the value in quotes as Guy shows. (JavaScript has two kinds of quotes, ' and "; you only need to escape the kind you use around the value.) Those kinds of assumptions tend to break down, though; as soon as the string is Hi, I'm Joe that approach breaks.

In the general case, to ensure proper escaping and that all values are written correctly, you can use JSON:

import json
value = 'J590'
driver.execute_script(f"document.getElementById('CustCd').value = {json.dumps(value)};")

That outputs:

document.getElementById('CustCd').value = "J590";

Live Example

That way, you don't have to worry about quoting and escaping, it's all handled for you since valid JSON is valid JavaScript (thanks to a recent JavaScript specification fix; prior to that there was an edge case incompatibility that people almost never ran into).

It's also useful for numbers, or more complex things you might want to pass to hte JavaScript code. For instance:

import json

class Example:
    foo = ""
    bar = 0

    def __init__(self, foo, bar):
        self.foo = foo
        self.bar = bar

value = Example("I'm a string with \"quotes\" in it.", 42)
print(f"const obj = {json.dumps(value.__dict__)};")
num = 42
print(f"const num = {json.dumps(num)};")

That outputs:

const obj = {"foo": "I'm a string with \"quotes\" in it.", "bar": 42};
const num = 42;

obj ends up being an object, because the initializer is a valid JavaScript object literal containing the data from the Example object. Similarly, num is a valid JavaScript number.

Live Example