You are in luck! Go to your settings. Look for the section “Defaults “. You can select your default list, default fields (including start date), default Due Date, and default sort order.

But not from the new gMail add on. If I don't add the Start date using ~today, then it defaults to Never in RtM and I have no Start date. It would be better to have Start Date as one of the prompted for fields.

As suggested by @azclaire I have the default Due Date set to "Today" and when I open the RTM for GMail addon it automatically has "Today" in the Due Date field.
It looks as if the addon is picking up the Due Date from my settings.