Exhibit 10.19

JOINDER TO SPONSOR SUPPORT AND LOCK-UP AGREEMENT AND DEED

This JOINDER TO SPONSOR SUPPORT AND LOCK-UP AGREEMENT AND DEED (this “Joinder”) is made as of October 13, 2026, by and among Astrum Space Inc, an exempted company incorporated with limited liability under the laws of the Cayman Islands (the “Company”), Black Spade Acquisition III Co, an exempted company incorporated with limited liability under the laws of the Cayman Islands (“BIII”), Black Spade Sponsor LLC III, a limited liability company registered under the laws of the Cayman Islands (the “Sponsor”), and each of the other undersigned Person(s) (each, a “Joined Sponsor Party”).

RECITALS

WHEREAS, BIII, the Company and Astrum Networks Pte. Ltd., a private company limited by shares incorporated under the laws of Singapore and a wholly-owned subsidiary of the Company (“Astrum Singapore”), have entered into a Business Combination Agreement dated August 27, 2026 (as the same may be amended, restated or supplemented, the “Business Combination Agreement”) providing for, among other things, the merger of the Company with and into BIII, with BIII being the surviving entity and continuing its existence as a publicly traded company listed on the New York Stock Exchange (the “Merger”);

WHEREAS, in connection with the Business Combination Agreement, BIII, the Company, the Sponsor and certain other holders of Class B ordinary shares of BIII (“BIII Class B Ordinary Shares”) have entered into a Sponsor Support and Lock-up Agreement and Deed dated August 27, 2026, attached hereto as Annex A (“Sponsor Support and Lock-up Agreement”). Capitalized terms used but not defined in this Joinder shall have the meaning ascribed to such terms in the Sponsor Support and Lock-up Agreement; and

WHEREAS, pursuant to Section 5.6 of the Sponsor Support and Lock-up Agreement, the Sponsor and BIII are required, as promptly as practicable and in any event within 30 days following the date of the Sponsor Support and Lock-up Agreement, to exercise their respective reasonable best efforts to procure that the other holders of BIII Class B Ordinary Shares listed in Schedule A to the Sponsor Support and Lock-up Agreement who have not yet executed and delivered the Sponsor Support and Lock-up Agreement shall join the Sponsor Support and Lock-up Agreement as a “Sponsor Party”; and

WHEREAS, each Joined Sponsor Party desires to, subject to the terms and conditions of this Joinder, join the Sponsor Support and Lock-up Agreement as a “Sponsor Party”;

NOW, THEREFORE, in consideration of the mutual covenants and agreements herein contained, and for other good and valuable consideration the receipt and sufficiency of which are hereby acknowledged, the parties hereto, intending to be legally bound, agree as follows:

1. Joinder by Joined Sponsor Party. Each Joined Sponsor Party hereby acknowledges that such Joined Sponsor Party has received and reviewed a copy of the Sponsor Support and Lock-up Agreement and understands the terms thereof. Subject to the terms and conditions of this Joinder, each Joined Sponsor Party hereby agrees to, and does become a party to, the Sponsor Support and Lock-up Agreement as a “Sponsor Party”, and agrees to be and is bound by, and subject to, all of such terms and conditions thereof applicable to “Sponsor Party”, including all covenants, rights and obligations, to the same extent as if such Joined Sponsor Party were “Sponsor Party”, in each case effective as of the date hereof. This Joinder shall serve as a counterpart signature page to the Sponsor Support and Lock-up Agreement (as amended by this Joinder) and by executing below, the Joined Sponsor Party is deemed to have executed the Sponsor Support and Lock-up Agreement (as amended by this Joinder) as if an original party thereto, effective as of the date hereof. For the avoidance of doubt, as the lock-up restrictions set out in the Sponsor Support and Lock-up Agreement will supersede the lock-up restrictions contained in that certain letter agreements entered into by BIII, the Sponsor and each of the Sponsor Parties dated as of January 5, 2026, the Subject Shares held by each of the Joined Sponsor Parties shall be free from lock-up restrictions upon Closing.

2. Representations and Warranties of Joined Sponsor Party. Notwithstanding anything to the contrary in the Sponsor Support and Lock-up Agreement or this Joinder, the representations and warranties of the Sponsor Parties in the Sponsor Support and Lock-up Agreement that were made on the date thereof shall be deemed to be made as of the date hereof by each undersigned Joined Sponsor Party.

3. Miscellaneous. Section 5.7 (Notice) and Section 5.8 (Miscellaneous) of the Sponsor Support and Lock-up Agreement are incorporated herein by reference, mutatis mutandis, as if set forth in full herein.

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