See this tutorial on how to safely changes strings in Connections. Most strings in Form are shared from Connections, such as the one you wish to change. There are some Form specific strings. In that case you would use
connections_form as the textdomain. My suggestion would always try with the
connections textmain first and then try
connections_form if it has no effect.
Another option not mentioned that I quite like… Install the Loco Translate plugin. Make a new US English translation for both Connections and Form. You can then even more easily change strings in both because this plugin allows you to quickly search for strings that you want to translate (or change). If you go this route, you do not even need to keep Loco active after making the changes. Make sure you choose the System location so you changes are update safe.
Hope that help!