Automated test generation techniques improve the efficiency of software testing. However, the opacity of the test generation process and concerns about the readability of generated tests make it difficult for software developers to accept them. Developer-centric test amplification creates easy-to-understand test cases by amplifying existing test cases that developers are familiar with and assists developers in integrating them into their test suite. We propose user-guided test amplification to allow developers to guide the test amplification to generate new test cases based on their branch coverage expectations. We create a user-guided test amplification prototype that starts with the method developers want to test, aids developers in communicating which branch should be covered, and assists developers in inspecting and selecting the amplified test cases. We conduct a technical case study with two Java projects and show that our approach cannot always produce a test case to cover a given branch because objects are not initialized with the right parameter values to fulfill the target branch condition. We also perform a user study with 12 software developers to investigate developers' opinions on our approach. The evaluation result shows that the user-guided test amplification generates amplified test cases that developers are satisfied with and is especially useful when developers want to generate tests to cover a specific branch. Connecting the developers' coverage goal and the amplified test cases enables developers to understand and select the test cases more easily.