You can usually track the platform that the user came from. Could the platform pass through their handle? Start there - your default "Preferred Contact Method" dropdown is the platform, and the "Handle" is pre-populated with their handle. If the platform is passed through but the handle is not, still pre-populate the platform and let the user fill in the handle.
Having said that - please let the user choose a non-social contact method like email, too. They might not want to connect with companies in a space that they're using for friend and family connections. They might have a special burner email account they use for company outreach, to keep their real inbox free from marketing stuff. And they might prefer SMS contact (very popular with some demographic groups).