Need Help Positioning Search Field in Header


I have a header.php in my child theme as well as a custom.css. I wish to place the search field in the header and have a partial solution from the theme developer.

The theme developer is providing limited support on this item, and they explained to me to place the following code into my header.php just above the navigation HTML as shown here:

<div class="custom-search">
<?php get_search_form(); ?>

Since there is a DIV class, I can format this via my child custom.css, and I suppose I could also style it within the DIV with style="[parameters]" - but in either case, I'm not sure how to do the following:

1. Place the search field in the desired header position as shown in the attached image.

2. Ensure that the search field remains in position when the browser is displayed at various desktop sizes, or within a resized browser window. Is there a way to position it so that it remains relative, but in the proper position?

3. Notice that with the basic code inserted as I've already done, it pushes the main navigation left incorrectly.

This can all be observed at - our staging site. And I've opened support there.

Note: once this is position/styled correctly, I already have a means to then make the header sticky.

Thank you sincerely for any help!