To add a button:

1. Press Ctrl + Shift + P (or select Edit > Preferences).
2. In the left panel, select Filter Expressions.
3. Select the plus icon.
4. Give the button a label. The label is what will appear int the toolbar in Wireshark.
5. Select a filter expression.
6. Select OK.

In this example, a few buttons have been added (TCP Delta Time, HTTP Time, DNS Time).