Yes, it's perfectly fine to use the Missouri address for both of you on the MO-NRI form. For your second question, most practitioners recommend entering "FO" (for Foreign) or "UK" in that box.
If the software blocks "UK": If the software requires a 2-letter U.S. state code and won't let you leave it blank, you should use "ZZ" (the standard code for foreign/other) or simply your own state (MO).
The purpose of that box is to tell Missouri which other jurisdiction has the right to tax that income. Since the UK isn't a U.S. state, "ZZ" or "FO" is the most accurate. If the software requires you to choose from the dropdown, pick Missouri—even if it doesn’t seem right. Then, on the Income Allocation screen of the MO-NRI, set your spouse’s Missouri income to 0%.
One final note. Since you are filing a federal MFJ election for the first time, Missouri requires you to attach a copy of your Federal Return. If you are e-filing, the software usually attaches this automatically. If you end up having to paper-file due to the foreign address issue, make sure to include a copy of that signed election statement you prepared for the IRS.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
