https://www.theverge.com/2019/2/20/18233819/samsung-galaxy-s10-bixby-button-remap-reassign

Samsung will allow buyers of its new Galaxy S10 smartphones to use the Bixby key to open whatever app they want. The Verge has confirmed that with the S10, the company has added settings that allow you to customize the button beyond what has been allowed previously. You can assign the Bixby button to open a third-party app with either a single or double press of the button.