Generally speaking, the letter of recommendation should present you, as a candidate, to your potential employer. As such, it should be from someone who:
- Knows you in a professional capacity.
- Knows you in the capacity which is (at least partly) relevant to the position you apply for.
To successfully learn on the master program, the candidate shall have a certain set of skills (e.g. knowledge of a particular subject), but also soft skills (commitment, dedication, self-motivation, teamwork, responsibility, ability to work independently, leadership, etc.). If the head of your professional organisation can emphasise these attributes in their letter, and justify them using examples and evidence, it is a (very) good idea to have the recommendation letter from them.