Use Arc/Fillet,L,L with the two line references and the required radius. Then use Trim operators to remove the corner material.
Full Answer
Creating a fillet is a two-step process: 1) Create the fillet arc with Arc/Fillet,L,L. 2) Trim the corners with Trim/L,to,A.
Always create all fillet arcs before trimming. Trimming changes line endpoints which may affect subsequent fillet calculations.
XyzPilot Example
L0 = Horizontal line at Y=0
L1 = Vertical line at X=100
Step 1: Arc/Fillet, L0, L1, R=10
→ A0: centre=(90,10), radius=10
Step 2: Trim/L0, to, A0, End → L0 trimmed to (90,0)
Step 3: Trim/L1, to, A0, Start → L1 trimmed from (100,10)