In the Geogebra 6.0.579 version, when you click on the slider tool, Slider dialog box appears.

In the Name field, you will see a=1.

Place the cursor in the Name field, and a virtual key broad becomes active.

From the key board select αβγ option. Greek symbol keys are seen. Please select theta from the keys.

Similarly, In the left panel you can see an input bar with a + symbol.

To insert symbols click in the input bar. Again virtual key board becomes active.

From the key board select αβγ option. Greek symbols keys are seen. Please select theta from the keys.