Selecting form field

I included in the project a javascript function that sets the desired field in the form, but regardless of the fact that the function is executed (which is shortly visible), before the actual data entry, the active field is again the first field in the form. What did I miss?

I don't understand your question. Please clarify. What type of component are you using? What is the JavaScript function? Why are you using JavaScript at all? What are you trying to do?

I have RadzenForm and on some occasions I need to select not the first field in a form but second. So I implement this

        protected override async Task OnAfterRenderAsync(bool firstRender)
        {
            if (firstRender)
            {
                  await JSRuntime.InvokeVoidAsync("setFocusOnElement", "OPIS");
            }
        }

In App.razor I put

    <script>
        function setFocusOnElement(id) {
            setTimeout(function () {
                document.getElementById(id).focus();
            }, 0);
        }
    </script>

I expected after opening the form (dialog) to have the field "OPIS" selected but it is not what happened.
First field, which is set to be read only, was in focus.

Do you mean to focus? All components have a built-in method FocusAsync. You can try using it instead:

if (firstRender) 
{
    await myTextBox.FocusAsync();
}