How do I tie notes together?

Select the note (or the stem) and press T

A "bar line" is a radically different thing than a "tie". You would not "tie notes together using the bar line". Instead you would tie notes with the tie symbol. Select the first note of the pair (of identical notes!!!) to be tied. Click the "Tie" button on the toolbar ...

... or press the T key (assuming you have not changed the default shortcuts).

If you wish to tie two different pitched notes together, this is not a "tie": it is referred to as a "slur". The button is the one immediately to the right of the "tie" button in that screenshot and the (default!) shortcut is S.

To tie the notes of a chord to the (same) notes of another chord, click one of the notes in the first chord to select it, then Ctrl+click each of the others to add them to the selection, then click the button or press T.

"MUSESCORE 4 does not let me tie notes (specifically chords) together using the bar line."

Are you referring to ties across a repeat barline? Perhaps the newly-released MuseScore Studio 4.5 can help?

See the new Handbook article Items across repeats and jumps which describes this behaviour specifically for ties, slurs and lyrics lines:
https://handbook.musescore.org/notation/repeats/items-across-repeats-an…