All you have to do is:

1. Drag the Interaction Use to the sequence diagram
2. Click on the Interaction Use placed in the diagram
3. In the menu located in the lower right corner of StarUml called "Editors", in the properties group, in the refersTo field, click on the magnifying glass and select the other sequence diagram in your project. But in fact, you will not select the sequence diagram you want itself, you will select the element immediately above the sequence diagram you want to reference, which is represented by an orange icon with two arrows, one pointing to the right and the other to the left, which comes with the default name called "Interaction1"