What is the optimal pipeline for updating table on FK value in Postgresql?

Question

Hi I have the following schema in my app.

-- Table: Investment_round
CREATE TABLE Investment_round (
    id serial  NOT NULL,
    round_type varchar(50)  NOT NULL,
    money bigint  NOT NULL,
    Project_id int  NOT NULL,
    CONSTRAINT Investment_round_pk PRIMARY KEY (id)
);

-- Table: Investor
CREATE TABLE Investor (
    id serial  NOT NULL,
    name int  NOT NULL,
    url int  NULL,
    Investment_round_id int  NOT NULL,
    CONSTRAINT Investor_pk PRIMARY KEY (id)
);

-- Reference: Investment_round_startups_page (table: Investment_round)
ALTER TABLE Investment_round ADD CONSTRAINT Investment_round_startups_page
    FOREIGN KEY (Project_id)
    REFERENCES startups_page (angel_id)  
    NOT DEFERRABLE 
    INITIALLY IMMEDIATE
;

-- Reference: Investor_Investment_round (table: Investor)
ALTER TABLE Investor ADD CONSTRAINT Investor_Investment_round
    FOREIGN KEY (Investment_round_id)
    REFERENCES Investment_round (id)  
    NOT DEFERRABLE 
    INITIALLY IMMEDIATE
;

I'm a bit confused about optimal pipeline of inserting the data. Suppose that firstly I need to insert the data to Investment_round table. Then I need to insert the data to Investor table with corresponding Investment_round_id

It seems that the reasonable way is to store Investment_round_id and then to update Investor table. But how can I achieve that without making an external request?