you can try using io.github.sukgu that helps you to work on the shadow elements. I was able to automate the scenario that you mentioned. Below is the detailed code.
Step 1 add the below dependency
<!-- https://mvnrepository.com/artifact/io.github.sukgu/automation -->
<dependency>
<groupId>io.github.sukgu</groupId>
<artifactId>automation</artifactId>
<version>0.1.3</version>
</dependency>
Step 2 use the below import in the test file
import io.github.sukgu.*;
Step 3 Below is the entire code that worked for me
WebDriver driver = new ChromeDriver();
driver.get("<your_webpage>");
Shadow shadow = new Shadow(driver);
WebElement searchField = shadow.findElement("input[placeholder='Search']");