To Add a FixedPoint Constraint

In the Geometric Constraints toolbox, select the Fix Point at Location tool.
 Snap to the profile at the outer corner of the two legs of the L.

Accept to place the fixed point constraint.
In the status bar, the message DOF=4 indicates that the profile has 4 degrees of freedom.
With the fixed point constraint in place, and knowing that the profiles has 4 degress of freedom (DOF), you will add dimensional constraints. You will notice that each of the remaining "unfixed" vertices has a yellow marker and question mark (you mau need to zoom in to clearly see the yellow question marks). These indicate points that are not fixed. The vertex with the fixed point constraint has a white marker and no question mark.
You will fix the other points, and reduce the degress of freedom to zero, by adding dimensional constraints to the profile. These can be added in either of two ways. You can dimension the profile and then use the Convert Dimension to Constraint tool to convert the dimensions to constraints. Alternatively, you can specify the constraints as you add the dimensions. Here, are steps that use the latter option.