Note: Defaults are only possible with single-element dialogs. They do not work with "--forms"!
zenity --entry --title="Choose" --text "Option" --entry-text "optionA" optionA optionB optionC
Pass the possible values as additional parameters and the default with "--entry-text"
Use parameter "--forms"
zenity --forms --title="title" \
--text="Parameters" \
--add-entry="First param" \
--add-entry="Second param"
Returns value on stdout separated by a pipe "<param 1>|<param 2>"