ForeFlight allows you to add a call sign to your flight plan. Here is a helpful video that describes how to do this:
Video 1: How to file with a call sign.
The setting that allows for filing a call sign is disabled by default. You can enable this option on the MORE page under SETTINGS > FILE & BRIEF. You will need to turn the switch on next to the ENABLE CALL SIGN field. This will cause a Call Sign field to appear in the flight plan form in the Aircraft section.
Image 1. Turn on the Call Sign field under MORE > SETTINGS > FILE & BRIEF.
To file under your call sign, enter your call sign using the contraction and number in the Call Sign field. If your flight plan is filed using the domestic form, your tail number will be added to Remarks. If your flight plan is filed using the ICAO form, your tail number will be placed in field 18 after the REG/ label (registration number), for example REG/N12345. If you don’t enter a call sign, the flight will be filed under your N-number.
Image 2. Enter your call sign on the Call Sign field of the flight plan form.