Thanks for the answer. I figured it out.

I inherited the search view of res.partner and added my own filter like this:

<record id="base_view_res_partner_filter" model="ir.ui.view">

<field name="model">res.partner</field>

<field name="inherit_id" ref="base.view_res_partner_filter"/>

<field name="arch" type="xml">

<filter string="Salesperson" position="after">

<filter string="Investor" name="investor" domain="[('is_investor', '=', True)]"/>

</filter>

</field>

</record>

Then, in my corresponding action for 'investor' menu, I defined this line

<field name="context">{'search_default_investor':1}</field>

Note that the search_default_investor is named that way because the syntax is:

{'search_default_searchFilterName':1}

It should not be search_default_is_investor because is_investor is a field name not the name of a filter.

Someone reading this might also find this question useful:

https://www.odoo.com/forum/help-1/question/how-to-create-own-filter-and-apply-it-by-default-75280

Hope this helps someone. And thanks for the help Vasanth!