You want your text in perspective in Affinity Photo, a 3D effect against a wall or building.
Add some characters using the artistic text tool or the frame etc. With that selected, go to the layer menu and new live filter and distort and perspective.
You will see a bounding box appear with four points. You can then select those points and drag up and down and left and right and align the bounding to any perspective in the image. You can also set the show grid to be on which may make it easier to align the box. You can then close the panel.
You will see the layer entry now has a live filter added to it and they can be re-edited at any time by double clicking the entry. You can then edit the bounding box. If you wish to change the words or characters, I would suggest disabling the effect, though it should be noted that you may have to reset the dialog / bounding box as the result may not be what you expect.
It is possible to simply edit the characters by entering the insertion point and additional characters or removing from the word or words. You also delete the live effect at any time from the letters by selecting the entry in the layers panel and deleting it.
Create some letters ABC, you can set them in any font or size but big is probably best if you want them to match the size of a billboard or side of a building etc. Depending on the location of the building etc, place the characters to the top left of the area you want to cover. Apply live filter effect via the layer menu. Select the bottom left control point and drag down to the bottom of the area you want to use or cover. Go to the far bottom right control point and position that in the bottom right of the area you want to cover and now do the same with the top right. You may decide on a different order of manipulating the points but you just want to avoid twisting the bounding box as the result is not pretty nor can you move the points any further up or down once you have that twist in the bounding box (it doesn't auto flip for you)
You may find that if you apply some layer effects to the characters and then apply the live filter then the bevel etc all vanish, sometimes this happens, sometimes not. If this does happen, you can always select the characters and apply the fx such as bevel or shadow etc and then go to the layer menu and rasterize and deselect the fx check box and apply and then use the live filter layer. Once you have applied the live filter layer, you can always rasterize the result then as well via the layer menu and then add the shadow / bevel etc.
You can rasterize the result and then apply the filter again. You can also duplicate the live filter layer and set a different value of bounding box for that. Personally, not helpful if you are using this for alignment but useful if you wish to warp the letters in your design as you can also combine it with other live effects such as twirl etc