When filing married jointly with an incarcerated spouse, how do you get the signature from your spouse to be able to submit?

[KristinaK]

And to protect yourself against spousal disagreement, it is always a good idea to get either a Power of Attorney from your spouse allowing your to e-file the married filing jointly return; or at least, a written statement saying the same.

[Opus 17]

For the electronic signature, your spouse's verbal agreement would be fine in most cases.  If you are filing by mail and need their signature on paper, you will need to print the forms and take them to prison on visiting day, or send the forms by mail or express service and have your spouse sign them and mail them back to you.