How to approach admissions when applying for a Master's without Bachelor's degree?
Within the UK all degrees will have entry requirements for a course, and I expect this is the same throughout Europe although I suggest you check for yourself. As I dont know the exact situation for the entry, ie university or country, I will answer in general terms. 

Typical entry requrements are based on a progressive points system, where your previous qualification accrues you a certain number of points and these points can be used as validation of your suitability to be on the higher level course. Within the UK, we use UCAS Tarrif Points to acheive progress through the undergraduate (UG) phase and the UG degree award for suitability of Postgraduate progression. Note: Some UK Universities also offer access through the UCAS Postgraduate scheme, although this is not widely used by all institutions. 

In non-typical situations as yours where you do not have the preceding qualification level, so therefore, do not have the correct number of points or potentially the preceding degree award, you can demonstrate your suitability by evidencing that you have professinally worked at a very high level. Then usually, the university will conduct their own assessment of the qualifications you do have, the level of your subject knowledge, the relevance of your work experience and your academic potential. It might also be that you have to submit a pre-entry assessment of some sort, so a demonstration that you can perform to the epected academic level. This non-typical route is more common in situations where the applicant is considered a "Mature Student" ie has had time in the workplace and is returning to education. 

So, to answer your question directly, I would contact the admissions office of the relevant universities you are interested in and be clear that you would have to take a non-typical admission route, and enquire directly as to what their requirements would be in this regard. It might help to have examples of your work already in mind, case studies perhaps, that you can discuss with admissions, but bear in mind they will be non-specialists and will have to refer your query on to the course managers or similar. 

HTH.