Try this:

Let us say that you need the chord symbol Dmi7.

While you are in the mode Type Into Score, type D:0 (the number zero, not the letter o), and hit Return.

You get to the dialog box Chord Suffix Selection where you can choose the suffix, you need.

Note that each suffix has a number.

Example:

In the default file Maestro Font Default the suffix mi7 has the number 150.

When you know a suffix’ number, you can enter the suffix by typing colon followed by the suffix number.

To enter Dmi7, type D:150.

By The Way, something else to try:

Instead  of using Type Into Score, double-click on the staff.

You get to the dialog box Chord Definition where you can choose a chord root, and select a suffix.

To select a suffix, click the button “Select…” at the bottom of the dialog box.

You get to a different version of the Chord Suffix Selection where there are buttons so that you can edit each suffix, or create a new suffix.