Call the Capitol Switchboard 202-224-3121 and ask for your Senators. You will need to call twice to reach each of your US Senators.
If you need help finding your Senators, you can ask the Capitol Switchboard attendant by telling them what state you reside or use this link.
Once you are connected, ask to leave a time-sensitive Stimulus Package message for your Senator.
"Thank you for taking my call. My address is______________ and I live in the Senator's state. Please tell your boss, that the current Stimulus Package should include international development assistance, specifically debt relief measures that are supported by the White House and US Treasury for the 73 poorest countries of the world. In addition, please support measures that can cancel debt and support accountable reporting to Congress on how debt relief helps these 73 developing countries address the severe health and economic impacts of the coronavirus crisis. Thank-you."