Use Capital letters in your Pattern or Letters group to require that letter to be in the results.
The Sort = option lists the words by score, length or alphabetically.
To fit words into already played letters on the game board, use the Pattern option. This finds words that can be fit onto already played ones using your rack letters.
Input the pattern as a mix of existing letters and open squares on the play board. Input open square positions as the '_' or '?' character.
For example, if you're trying to fit a word across an area that has 1st letter A and 3rd letter P, type in 'A?P' for the pattern. Note that using capital letters requires them to be in the answers.
The three circles after the Pattern box indicate where to match the string. They act as follows in order: Start, Anywhere or End.
The Minimum Word Length option shows words at least that long or larger.