Use Arc/Fillet,L,L to create a fillet arc tangent to two lines. Specify the two lines and the required radius.
Full Answer
Use Arc/Fillet,L,L. The arc is tangent to both lines simultaneously. After creating the fillet arc, use Trim/L,to,A to remove the corner material from the original lines.
XyzPilot Example
L0 = Horizontal line at Y=0
L1 = Vertical line at X=100
Arc/Fillet, L0, L1, R=15
→ A0: centre=(85,15), radius=15
Start: (85,0) tangent to L0
End: (100,15) tangent to L1
Trim/L0, to, A0, End → L0 trimmed to (85,0)
Trim/L1, to, A0, Start → L1 trimmed from (100,15)