How to add a "selection function" to a visualization"

Question

I tried to add a selection to a visualization i am doing. I tried everything with altair (it must be altair).

alt.Chart(Data1990).mark_circle().encode(
    x='Annual net change in forest area:Q',
    tooltip=['Year', 'Annual net change in forest area', 'Entity'],
    color='Entity:N'
).properties(
    width=600,
    height=400,
    title='Annual Net Change in Forest Area by Country'
).interactive()

Here is the code i am using at the moment. I wanted to add a selection for countrys. I have no idea how to do it. Does anyone have a tip for me?