js assign new input value to form object without enforcing datetime-local format

The datetime-local html input type accepts a value in the format 2021-12-04T18:45, however my app uses dates in the 2021-12-04 18:45 format, so I’m trying to change the value of the datetime-local input with JavaScript on form submit:

document.getElementById('myform').onsubmit = (event) => {
    let _form = event.target;
    _form.elements.namedItem('date_from').value = '2021-12-10 20:50';
};

but get the following error:

The specified value “2021-12-10 20:50” does not conform to the
required format. The format is “yyyy-MM-ddThh:mm” followed by
optional “:ss” or “:ss.SSS”.

How do I bypass the formatting enforcement in this scenario?

I’d prefer using datetime-local input type, to take advantage of the built-in date and time picker as opposed to having to use extra library. So changing the input field type would not be a preferable solution.

P.S. I cannot control the backend processing of the form, so I have to change the value in the frontend before the form is submitted further.