RCPTT support of Internal browser is very limited – basically RCPTT can only record keytypes in it, wait for URL loading and asserting HTML text.

However if your interaction with internal browser is very limited, then it is sort of possible to interact with it by calling org.eclipse.swt.widgets.Browser#evaluate method directly from ECL by using get-object and invoke commands, like this:

with [... | get-control Browser] {
    get-object | invoke evaluate "document.getElementById('searchBox').value='hello, world!';\n"+
                                 "document.getElementById('searchButton').click();"
}