Problem:

How do I assign a build and version to a Test definition when executing Unassigned Tests? (i.e. - Trial Run of a Test Definition)

Resolution:

Unassigned tests are created by "Right Clicking" on a test definition and choosing "Try Run"
See Example below:-

1. Right-click a Test definition and choose to Trial Run a test definition

2. In the Manual Test Execution screen you will notice that the test definition will be labelled “Unassigned Tests (Build Number / Version Number)

Currently there is no way to specify a build or version when performing a try run.

Explanation:-

Try run takes the build and version from the latest execution for a specified product regardless of the Test Definition, Execution Definition and Project this run was for. SCTM searches through all results (of all projects) having the same product as the "Try run" test and takes the build of the last one.

If it was the first run for this product (in all projects) the latest active build of the product is taken.

Please note that the "Try Run" is only designed for initial trial tests.