The key of B major is

B, C#, D#, E, F#, G#, A#, (B)

For B major triad chord we take root, the third and the fifth.

B - D# - F#

To make it (dominant) 7th chord we add flat seventh to the triad.

Seventh is A#, so flat seventh is A.

B7 = B - D# - F# - A

Order of the notes doesn't matter, as long it contains those notes it is B7.

Open B7

e| --- 2 ---

B| --- 0 ---

G| --- 2 ---

D| --- 1 ---

A| --- 2 ---

E| --- X ---

Same using note names

e| --- F# ---

B| --- B ---

G| --- A ---

D| --- D# ---

A| --- B ---

E| --- X ---