I doubt that there is a particular technique for the very first word of a text. If you think about it, it may generally not be feasible for authors to include literary techniques in the very first few words/sentences of their texts, especially since there may not be much development and/or significant events/aspects being explored at a rather early stage in the texts.
In the case of a poem, a technique that can be considered slightly similar to the one that you have described would be an acrostic, which is where the first word (or first letter/syllable) of every verse consecutively forms a word or message. However, this applies to an entire poem instead of one word or the first word, meaning that this would not be a useful technique for the purposes of the poem that you are analysing.
With that being said, you should still be able to discuss the fact that the word "when" in the poem invokes ideas of limited time and such without necessarily mentioning a specific technique in this instance, especially if this point is evidently consistent with the rest of your analysis. You could also link it to the overall structure/form as you have suggested, which I believe would be appropriate, since form essentially affects readers, appeals to their senses and elicits an emotional response, which is in turn evident when considering the notion of concerns as you mentioned. Here, one could argue that form acts as a means of facilitating the delivery of a key message or (and perhaps more relevant to your poem) raising key ideas/themes/issues.
I hope this helps!
